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/analysis/ConfiguredTarget.java b/src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTarget.java
index 995c2b9..155110c 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTarget.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/ConfiguredTarget.java
@@ -30,6 +30,18 @@
  * accessible from the action graph.
  */
 public interface ConfiguredTarget extends TransitiveInfoCollection {
+
+  /**
+   *  All <code>ConfiguredTarget</code>s have a "label" field.
+   */
+  String LABEL_FIELD = "label";
+
+  /**
+   *  All <code>ConfiguredTarget</code>s have a "files" field.
+   */
+  String FILES_FIELD = "files";
+
+
   /**
    * Returns the Target with which this {@link ConfiguredTarget} is associated.
    */