Refactor get_local_defns.

When getting function annotations for normal functions:

• in get_local_defns
  • Create a duplicate function with the computed annotations
  • Pass that function out in dct
• in get_annotated_method_hints
  • Get the signature from that function
  • Generate a function type

Since this is the only use of dct, we can do all that work in get_local_defns by generating the signature directly.

Reusing the _resolved_function_signature for all types of functions also makes things easier.