Python: Bring back support for flow-summaries

Also needed to fix up `TestUtil/UnresolvedCalls.qll` after a bad merge
conflict resolution. Since all calls are now DataFlowCall, and not JUST
the ones that can be resolved, we need to put in the restriction that
the callable can also be resolved.

python/ql/test/experimental/dataflow/TestUtil/UnresolvedCalls.qll

@@ -12,7 +12,9 @@ class UnresolvedCallExpectations extends InlineExpectationsTest {
   override predicate hasActualResult(Location location, string element, string tag, string value) {
     exists(location.getFile().getRelativePath()) and
     exists(CallNode call |
-      not exists(DataFlowPrivate::DataFlowCall dfc | dfc.getNode() = call) and
+      not exists(DataFlowPrivate::DataFlowCall dfc |
+        exists(dfc.getCallable()) and dfc.getNode() = call
+      ) and
       not DataFlowPrivate::resolveClassCall(call, _) and
       not call = API::builtin(_).getACall().asCfgNode() and
       location = call.getLocation() and