Python: support callbacks to library calls

TODO:
The member predicate `LibraryLambdaMethod::getACall` is
currently too permissive.
Ideally, we would have `libraryCallHasLambdaArg`
as in Ruby. But even a more precise
`libraryCall` predicate might be fine.

python/ql/test/experimental/dataflow/variable-capture/test_library_calls.py

@@ -45,4 +45,4 @@ def test_library_call():
     for x in map(set, [1]):
         pass
 
-    SINK(captured["x"]) #$ MISSING:captured
+    SINK(captured["x"]) #$ captured