diff --git a/src/main/java/com/google/devtools/build/lib/packages/Package.java b/src/main/java/com/google/devtools/build/lib/packages/Package.java
index dd0df6a..8a666b0 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/Package.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/Package.java
@@ -209,7 +209,7 @@
    * @precondition {@code name} must be a suffix of
    * {@code filename.getParentDirectory())}.
    */
-  private Package(PackageIdentifier packageId, String runfilesPrefix) {
+  protected Package(PackageIdentifier packageId, String runfilesPrefix) {
     this.packageIdentifier = packageId;
     this.workspaceName = runfilesPrefix;
     this.nameFragment = Canonicalizer.fragments().intern(packageId.getPackageFragment());
@@ -531,13 +531,23 @@
       suffix = "";
     }
 
+    throw makeNoSuchTargetException(targetName, suffix);
+  }
+
+  protected NoSuchTargetException makeNoSuchTargetException(String targetName, String suffix) {
+    Label label;
     try {
-      throw new NoSuchTargetException(createLabel(targetName), "target '" + targetName
-          + "' not declared in package '" + name + "'" + suffix + " defined by "
-          + this.filename);
+      label = createLabel(targetName);
     } catch (LabelSyntaxException e) {
       throw new IllegalArgumentException(targetName);
     }
+    String msg = String.format(
+        "target '%s' not declared in package '%s'%s defined by %s",
+        targetName,
+        name,
+        suffix,
+        filename);
+    return new NoSuchTargetException(label, msg);
   }
 
   /**
@@ -666,16 +676,36 @@
     }
   }
 
-  public static Builder newExternalPackageBuilder(Path workspacePath, String runfilesPrefix) {
-    Builder b = new Builder(Label.EXTERNAL_PACKAGE_IDENTIFIER, runfilesPrefix);
+  public static Builder newExternalPackageBuilder(Builder.Helper helper, Path workspacePath,
+      String runfilesPrefix) {
+    Builder b = new Builder(helper.createFreshPackage(
+        Label.EXTERNAL_PACKAGE_IDENTIFIER, runfilesPrefix));
     b.setFilename(workspacePath);
     b.setMakeEnv(new MakeEnvironment.Builder());
     return b;
   }
 
+  /** A builder for {@link Package} objects. Only intended to be used by {@link PackageFactory}. */
   public static class Builder {
-    protected static Package newPackage(PackageIdentifier packageId, String runfilesPrefix) {
-      return new Package(packageId, runfilesPrefix);
+    public static interface Helper {
+      /**
+       * Returns a fresh {@link Package} instance that a {@link Builder} will internally mutate
+       * during package loading.
+       */
+      Package createFreshPackage(PackageIdentifier packageId, String runfilesPrefix);
+    }
+
+    /** {@link Helper} that simply calls the {@link Package} constructor. */
+    public static class DefaultHelper implements Helper {
+      public static final DefaultHelper INSTANCE = new DefaultHelper();
+
+      private DefaultHelper() {
+      }
+
+      @Override
+      public Package createFreshPackage(PackageIdentifier packageId, String runfilesPrefix) {
+        return new Package(packageId, runfilesPrefix);
+      }
     }
 
     /**
@@ -740,8 +770,8 @@
       }
     }
 
-    public Builder(PackageIdentifier id, String runfilesPrefix) {
-      this(newPackage(id, runfilesPrefix));
+    public Builder(Helper helper, PackageIdentifier id, String runfilesPrefix) {
+      this(helper.createFreshPackage(id, runfilesPrefix));
     }
 
     protected PackageIdentifier getPackageIdentifier() {
diff --git a/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java b/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
index b1ad355..73ff55d 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/PackageFactory.java
@@ -331,6 +331,8 @@
   private final ImmutableList<EnvironmentExtension> environmentExtensions;
   private final ImmutableMap<String, PackageArgument<?>> packageArguments;
 
+  private final Package.Builder.Helper packageBuilderHelper;
+
   /**
    * Constructs a {@code PackageFactory} instance with the given rule factory.
    */
@@ -341,7 +343,8 @@
         null,
         AttributeContainer.ATTRIBUTE_CONTAINER_FACTORY,
         ImmutableList.<EnvironmentExtension>of(),
-        "test");
+        "test",
+        Package.Builder.DefaultHelper.INSTANCE);
   }
 
   @VisibleForTesting
@@ -352,7 +355,8 @@
         null,
         AttributeContainer.ATTRIBUTE_CONTAINER_FACTORY,
         ImmutableList.of(environmentExtension),
-        "test");
+        "test",
+        Package.Builder.DefaultHelper.INSTANCE);
   }
 
   @VisibleForTesting
@@ -363,7 +367,8 @@
         null,
         AttributeContainer.ATTRIBUTE_CONTAINER_FACTORY,
         environmentExtensions,
-        "test");
+        "test",
+        Package.Builder.DefaultHelper.INSTANCE);
   }
 
   /**
@@ -375,7 +380,8 @@
       Map<String, String> platformSetRegexps,
       Function<RuleClass, AttributeContainer> attributeContainerFactory,
       Iterable<EnvironmentExtension> environmentExtensions,
-      String version) {
+      String version,
+      Package.Builder.Helper packageBuilderHelper) {
     this.platformSetRegexps = platformSetRegexps;
     this.ruleFactory = new RuleFactory(ruleClassProvider, attributeContainerFactory);
     this.ruleClassProvider = ruleClassProvider;
@@ -388,6 +394,7 @@
     this.packageArguments = createPackageArguments();
     this.nativeModule = newNativeModule();
     this.workspaceNativeModule = WorkspaceFactory.newNativeModule(ruleClassProvider, version);
+    this.packageBuilderHelper = packageBuilderHelper;
   }
 
   /**
@@ -1276,14 +1283,24 @@
   }
 
   @VisibleForTesting
+  public Package.Builder newExternalPackageBuilder(Path workspacePath, String runfilesPrefix) {
+    return Package.newExternalPackageBuilder(packageBuilderHelper, workspacePath, runfilesPrefix);
+  }
+
+  @VisibleForTesting
+  public Package.Builder newPackageBuilder(PackageIdentifier packageId, String runfilesPrefix) {
+    return new Package.Builder(packageBuilderHelper, packageId, runfilesPrefix);
+  }
+
+  @VisibleForTesting
   public Package createPackageForTesting(
       PackageIdentifier packageId,
       Path buildFile,
       CachingPackageLocator locator,
       EventHandler eventHandler)
       throws NoSuchPackageException, InterruptedException {
-    Package externalPkg =
-        Package.newExternalPackageBuilder(buildFile.getRelative("WORKSPACE"), "TESTING").build();
+    Package externalPkg = newExternalPackageBuilder(
+        buildFile.getRelative("WORKSPACE"), "TESTING").build();
     return createPackageForTesting(packageId, externalPkg, buildFile, locator, eventHandler);
   }
 
@@ -1533,8 +1550,8 @@
       Map<String, Extension> imports,
       ImmutableList<Label> skylarkFileDependencies)
       throws InterruptedException {
-    Package.Builder pkgBuilder = new Package.Builder(
-        packageId, ruleClassProvider.getRunfilesPrefix());
+    Package.Builder pkgBuilder = new Package.Builder(packageBuilderHelper.createFreshPackage(
+        packageId, ruleClassProvider.getRunfilesPrefix()));
     StoredEventHandler eventHandler = new StoredEventHandler();
 
     try (Mutability mutability = Mutability.create("package %s", packageId)) {
@@ -1621,8 +1638,8 @@
           .setPhase(Phase.LOADING)
           .build();
 
-      Package.Builder pkgBuilder = new Package.Builder(packageId,
-          ruleClassProvider.getRunfilesPrefix());
+      Package.Builder pkgBuilder = new Package.Builder(packageBuilderHelper.createFreshPackage(
+          packageId, ruleClassProvider.getRunfilesPrefix()));
 
       pkgBuilder.setFilename(buildFilePath)
           .setMakeEnv(pkgMakeEnv)
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/BlazeModule.java b/src/main/java/com/google/devtools/build/lib/runtime/BlazeModule.java
index 25f692a..743cff5 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/BlazeModule.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/BlazeModule.java
@@ -28,6 +28,7 @@
 import com.google.devtools.build.lib.exec.OutputService;
 import com.google.devtools.build.lib.packages.AttributeContainer;
 import com.google.devtools.build.lib.packages.NoSuchThingException;
+import com.google.devtools.build.lib.packages.Package;
 import com.google.devtools.build.lib.packages.PackageFactory;
 import com.google.devtools.build.lib.packages.Preprocessor;
 import com.google.devtools.build.lib.packages.RuleClass;
@@ -342,6 +343,15 @@
   }
 
   /**
+   * Returns a helper that the {@link PackageFactory} will use during package loading. If the module
+   * does not provide any heloer, it should return null. Note that only one helper per Bazel/Blaze
+   * runtime is allowed.
+   */
+  public Package.Builder.Helper getPackageBuilderHelper() {
+    return null;
+  }
+
+  /**
    * Returns a factory for creating {@link AbstractBlazeQueryEnvironment} objects.
    * If the module does not provide any {@link QueryEnvironmentFactory}, it should return null. Note
    * that only one factory per Bazel/Blaze runtime is allowed.
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java b/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java
index 8c9af36..ce6b1f5 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/BlazeRuntime.java
@@ -39,6 +39,7 @@
 import com.google.devtools.build.lib.events.OutputFilter;
 import com.google.devtools.build.lib.flags.CommandNameCache;
 import com.google.devtools.build.lib.packages.AttributeContainer;
+import com.google.devtools.build.lib.packages.Package;
 import com.google.devtools.build.lib.packages.PackageFactory;
 import com.google.devtools.build.lib.packages.Preprocessor;
 import com.google.devtools.build.lib.packages.RuleClass;
@@ -1260,13 +1261,27 @@
         extensions.add(module.getPackageEnvironmentExtension());
       }
 
+      Package.Builder.Helper packageBuilderHelper = null;
+      for (BlazeModule module : blazeModules) {
+        Package.Builder.Helper candidateHelper = module.getPackageBuilderHelper();
+        if (candidateHelper != null) {
+          Preconditions.checkState(packageBuilderHelper == null,
+              "more than one module defines a package builder helper");
+          packageBuilderHelper = candidateHelper;
+        }
+      }
+      if (packageBuilderHelper == null) {
+        packageBuilderHelper = Package.Builder.DefaultHelper.INSTANCE;
+      }
+
       PackageFactory packageFactory =
           new PackageFactory(
               ruleClassProvider,
               platformRegexps,
               attributeContainerFactory,
               extensions,
-              BlazeVersionInfo.instance().getVersion());
+              BlazeVersionInfo.instance().getVersion(),
+              packageBuilderHelper);
 
       if (configurationFactory == null) {
         configurationFactory = new ConfigurationFactory(
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/WorkspaceFileFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/WorkspaceFileFunction.java
index c84e420..87ec20c 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/WorkspaceFileFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/WorkspaceFileFunction.java
@@ -67,8 +67,8 @@
     }
 
     Path repoWorkspace = workspaceRoot.getRoot().getRelative(workspaceRoot.getRelativePath());
-    Package.Builder builder =
-        Package.newExternalPackageBuilder(repoWorkspace, ruleClassProvider.getRunfilesPrefix());
+    Package.Builder builder = packageFactory.newExternalPackageBuilder(
+        repoWorkspace, ruleClassProvider.getRunfilesPrefix());
 
     if (workspaceASTValue.getASTs().isEmpty()) {
       return new WorkspaceFileValue(