Python: Simple dataflow annotations

None of these required any changes to the dataflow libraries, so it
seemed easiest to put them in their own commit.

python/ql/lib/semmle/python/internal/Awaited.qll

@@ -3,6 +3,8 @@
  *
  * Provides helper class for defining additional API graph edges.
  */
+overlay[local]
+module;
 
 private import python
 private import semmle.python.dataflow.new.DataFlow

python/ql/lib/semmle/python/internal/CachedStages.qll

@@ -177,6 +177,7 @@ module Stages {
      * Always holds.
      * Ensures that a predicate is evaluated as part of the DataFlow stage.
      */
+    overlay[local]
     cached
     predicate ref() { 1 = 1 }