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/main/java/com/google/devtools/build/lib/cmdline/TargetPattern.java b/src/main/java/com/google/devtools/build/lib/cmdline/TargetPattern.java
index 610a27e..13e5578 100644
--- a/src/main/java/com/google/devtools/build/lib/cmdline/TargetPattern.java
+++ b/src/main/java/com/google/devtools/build/lib/cmdline/TargetPattern.java
@@ -644,7 +644,7 @@
       }
 
       if (repository == null) {
-        repository = PackageIdentifier.MAIN_REPOSITORY_NAME;
+        repository = RepositoryName.MAIN;
       }
 
       if (packagePart.endsWith("/...")) {