Rename some PackageId and RepositoryName fields/methods in prep for deep execroot change

This is in prep for making the execution root path for external repositories
../repo_name (instead of external/repo_name). Right now, the getRunfilesPath() returns that path, so that is renamed getExecRoot() (since the runfiles are really just a reflection of the execRoot structure).  getSourceRoot() replaces getPathFragment, which has always been a confusing name (it's not clear from the name
what the difference is between it and getPackageFragment()). It returns the relative path to source files for external repositories (external/repo_name).

Also renamed/moved to more sensible class a few static RepositoryName fields.

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MOS_MIGRATED_REVID=128594419
diff --git a/src/main/java/com/google/devtools/build/lib/packages/InputFile.java b/src/main/java/com/google/devtools/build/lib/packages/InputFile.java
index 228af69..6f2fe22 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/InputFile.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/InputFile.java
@@ -104,7 +104,7 @@
    * Returns the exec path of the file, i.e. the path relative to the package source root.
    */
   public PathFragment getExecPath() {
-    return label.getPackageIdentifier().getPathFragment().getRelative(label.getName());
+    return label.getPackageIdentifier().getSourceRoot().getRelative(label.getName());
   }
 
   @Override