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.

--
MOS_MIGRATED_REVID=128594419
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/RecursivePkgFunctionTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/RecursivePkgFunctionTest.java
index 5a16bc3..d6c6459 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/RecursivePkgFunctionTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/RecursivePkgFunctionTest.java
@@ -22,7 +22,7 @@
 import com.google.common.collect.ImmutableSet;
 import com.google.common.collect.Iterables;
 import com.google.devtools.build.lib.analysis.util.BuildViewTestCase;
-import com.google.devtools.build.lib.cmdline.PackageIdentifier;
+import com.google.devtools.build.lib.cmdline.RepositoryName;
 import com.google.devtools.build.lib.util.Preconditions;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.build.lib.vfs.PathFragment;
@@ -60,7 +60,7 @@
       Path root, PathFragment rootRelativePath, ImmutableSet<PathFragment> excludedPaths) {
     RootedPath rootedPath = RootedPath.toRootedPath(root, rootRelativePath);
     return RecursivePkgValue.key(
-        PackageIdentifier.MAIN_REPOSITORY_NAME, rootedPath, excludedPaths);
+        RepositoryName.MAIN, rootedPath, excludedPaths);
   }
 
   private RecursivePkgValue buildRecursivePkgValue(Path root, PathFragment rootRelativePath)