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/RecursivePkgKeyTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/RecursivePkgKeyTest.java
index 6ee4ac0..9d0fc4d 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/RecursivePkgKeyTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/RecursivePkgKeyTest.java
@@ -17,7 +17,6 @@
 
 import com.google.common.collect.ImmutableSet;
 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.skyframe.RecursivePkgValue.RecursivePkgKey;
 import com.google.devtools.build.lib.vfs.PathFragment;
@@ -44,7 +43,7 @@
       PathFragment rootRelativePath, ImmutableSet<PathFragment> excludedPaths) {
     try {
       buildRecursivePkgKey(
-          PackageIdentifier.MAIN_REPOSITORY_NAME, rootRelativePath, excludedPaths);
+          RepositoryName.MAIN, rootRelativePath, excludedPaths);
       fail();
     } catch (IllegalArgumentException expected) {
     }
@@ -53,29 +52,29 @@
   @Test
   public void testValidRecursivePkgKeys() throws Exception {
     buildRecursivePkgKey(
-        PackageIdentifier.MAIN_REPOSITORY_NAME,
+        RepositoryName.MAIN,
         new PathFragment(""),
         ImmutableSet.<PathFragment>of());
     buildRecursivePkgKey(
-        PackageIdentifier.MAIN_REPOSITORY_NAME,
+        RepositoryName.MAIN,
         new PathFragment(""),
         ImmutableSet.of(new PathFragment("a")));
 
     buildRecursivePkgKey(
-        PackageIdentifier.MAIN_REPOSITORY_NAME,
+        RepositoryName.MAIN,
         new PathFragment("a"),
         ImmutableSet.<PathFragment>of());
     buildRecursivePkgKey(
-        PackageIdentifier.MAIN_REPOSITORY_NAME,
+        RepositoryName.MAIN,
         new PathFragment("a"),
         ImmutableSet.of(new PathFragment("a/b")));
 
     buildRecursivePkgKey(
-        PackageIdentifier.MAIN_REPOSITORY_NAME,
+        RepositoryName.MAIN,
         new PathFragment("a/b"),
         ImmutableSet.<PathFragment>of());
     buildRecursivePkgKey(
-        PackageIdentifier.MAIN_REPOSITORY_NAME,
+        RepositoryName.MAIN,
         new PathFragment("a/b"),
         ImmutableSet.of(new PathFragment("a/b/c")));
   }