Make the code that is using execRoot paths use execRoot paths

Chipping away at making the big CL for #1262 smaller. Only runfiles paths
are different right now, so this makes getPathUnderExecRoot and getSourceRoot
return the same thing. Also corrected a couple places where
Label.EXTERNAL_PATH_PREFIX and Label.EXTERNAL_PACKAGE_NAME were being used
incorrectly.

--
MOS_MIGRATED_REVID=132671081
diff --git a/src/test/java/com/google/devtools/build/lib/skyframe/FileFunctionTest.java b/src/test/java/com/google/devtools/build/lib/skyframe/FileFunctionTest.java
index 0269305..323fa84 100644
--- a/src/test/java/com/google/devtools/build/lib/skyframe/FileFunctionTest.java
+++ b/src/test/java/com/google/devtools/build/lib/skyframe/FileFunctionTest.java
@@ -319,7 +319,7 @@
   @Test
   public void testAbsoluteSymlinkToExternal() throws Exception {
     String externalPath =
-        outputBase.getRelative(Label.EXTERNAL_PATH_PREFIX).getRelative("a/b").getPathString();
+        outputBase.getRelative(Label.EXTERNAL_PACKAGE_NAME).getRelative("a/b").getPathString();
     symlink("a", externalPath);
     file("b");
     file(externalPath);