Shuffle around all the constants related to the name of the //external package and the prefix of external packages in the execroot.

This is in preparation for renaming that path segment from "external" to "bazel-external" so that we can provide a symlink to it from the source tree so that the source tree looks like the execroot.

--
MOS_MIGRATED_REVID=109882753
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 a9ad155..4dcd817 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
@@ -61,16 +61,6 @@
  */
 public class Package {
 
-  public static PackageIdentifier EXTERNAL_PACKAGE_IDENTIFIER;
-
-  static {
-    try {
-      Package.EXTERNAL_PACKAGE_IDENTIFIER = PackageIdentifier.parse("//external");
-    } catch (LabelSyntaxException e) {
-      throw new IllegalStateException();
-    }
-  }
-
   /**
    * Common superclass for all name-conflict exceptions.
    */
@@ -728,7 +718,7 @@
   }
 
   public static LegacyBuilder newExternalPackageBuilder(Path workspacePath, String runfilesPrefix) {
-    LegacyBuilder b = new LegacyBuilder(EXTERNAL_PACKAGE_IDENTIFIER, runfilesPrefix);
+    LegacyBuilder b = new LegacyBuilder(Label.EXTERNAL_PACKAGE_IDENTIFIER, runfilesPrefix);
     b.setFilename(workspacePath);
     b.setMakeEnv(new MakeEnvironment.Builder());
     return b;
@@ -811,7 +801,7 @@
 
     /** Determine if we are in the WORKSPACE file or not */
     public boolean isWorkspace() {
-      return pkg.getPackageIdentifier().equals(EXTERNAL_PACKAGE_IDENTIFIER);
+      return pkg.getPackageIdentifier().equals(Label.EXTERNAL_PACKAGE_IDENTIFIER);
     }
 
     /**