Always inject the runfiles prefix into the Runfiles.Builder.

The other constructor is now private.

--
MOS_MIGRATED_REVID=102252544
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 c6156ac..28681d4 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
@@ -761,6 +761,10 @@
   }
 
   static class Builder {
+    protected static Package newPackage(PackageIdentifier packageId) {
+      return new Package(packageId);
+    }
+
     /**
      * The output instance for this builder. Needs to be instantiated and
      * available with name info throughout initialization. All other settings
@@ -822,10 +826,9 @@
       }
     }
 
-    protected static Package newPackage(PackageIdentifier packageId) {
-      return new Package(packageId);
+    Builder(PackageIdentifier id) {
+      this(newPackage(id));
     }
-    Builder(PackageIdentifier id) { this(newPackage(id)); }
 
     protected PackageIdentifier getPackageIdentifier() {
       return pkg.getPackageIdentifier();