Use as_call_target to resolve calls for Expr::Name

Summary:
For now, this is used as a backup approach, when there is no go-to-definition.

For future:
- We still keep the callees computed from go-to-definitions. However later with
  more tests migrated, we may try to remove callees computed from
  go-to-definitions, because those may be less precise.
- We may also want to use `as_call_target` to resolve calls for
  `Expr::Attribute`.

Reviewed By: stroxler

Differential Revision: D84625457

fbshipit-source-id: fb6d5aa0ad7a604a611991478c4667a14bbf37d5

Author: Tianhan Lu

source/interprocedural/test/callGraphTest.ml

@@ -610,6 +610,7 @@ let test_call_graph_of_define =
            ();
       labeled_test_case __FUNCTION__ __LINE__
       @@ assert_call_graph_of_define
+           ~_is_migrated:true
            ~source:
              {|
     from typing import Protocol