Unify Skylark and BUILD lists

Use SkylarkList everywhere rather than either List or GlobList.
Keep a GlobList underneath a MutableList, where applicable.

--
MOS_MIGRATED_REVID=105864035
diff --git a/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java b/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
index f956f94..eb0f095 100644
--- a/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
+++ b/src/test/java/com/google/devtools/build/lib/syntax/SkylarkEvaluationTest.java
@@ -917,7 +917,7 @@
 
   @Test
   public void testListAnTupleConcatenationDoesNotWorkInSkylark() throws Exception {
-    new SkylarkTest().testIfExactError("can only concatenate list (not \"tuple\") to list",
+    new SkylarkTest().testIfExactError("unsupported operand type(s) for +: 'list' and 'tuple'",
         "[1, 2] + (3, 4)");
   }