Enable aspect invocations to be matched by output filters.

Currently aspects have no tag, i.e., they don't get checked against output
filters at all. This changes aspects to have a tag of the same form as the
rule they are attached to (i.e., the label of the target).

Since the default output filters match on ^//package[:/], this should cover
most uses of the output filter, while still allowing for finer control for
those who crave it.

--
MOS_MIGRATED_REVID=133425215
diff --git a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
index b988b4d..f160424 100644
--- a/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
+++ b/src/main/java/com/google/devtools/build/lib/analysis/RuleContext.java
@@ -1711,6 +1711,11 @@
       }
     }
 
+    /** Returns whether the context being constructed is for the evaluation of an aspect. */
+    public boolean forAspect() {
+      return aspectName != null;
+    }
+
     public Rule getRule() {
       return rule;
     }