Provide AspectDescriptor to ConfiguredAspect.

Also clean up the setting of aspect name in ConfiguredAspect and
AspectDefintion - it is now obtained from the AspectClass.

--
MOS_MIGRATED_REVID=140357052
diff --git a/src/main/java/com/google/devtools/build/lib/rules/AliasConfiguredTarget.java b/src/main/java/com/google/devtools/build/lib/rules/AliasConfiguredTarget.java
index 1c6eebd..a6363b7 100644
--- a/src/main/java/com/google/devtools/build/lib/rules/AliasConfiguredTarget.java
+++ b/src/main/java/com/google/devtools/build/lib/rules/AliasConfiguredTarget.java
@@ -117,9 +117,9 @@
 
   @Override
   public Object getValue(String name) {
-    if (name.equals("label")) {
+    if (name.equals(LABEL_FIELD)) {
       return getLabel();
-    } else if (name.equals("files")) {
+    } else if (name.equals(FILES_FIELD)) {
       // A shortcut for files to build in Skylark. FileConfiguredTarget and RuleConfiguredTarget
       // always has FileProvider and Error- and PackageGroupConfiguredTarget-s shouldn't be
       // accessible in Skylark.