Notify when Manager.from_queryset happens inside model class body (#824)

* Refactor to more easily support additional config options

* Notify when Manager.from_queryset happens inside model class body

- A warning will be emitted whenever `Manager.from_queryset` happens
  inside of a model class body

* Resolve generated default manager types before final iteration

A default manager on a model should always exist, eventually. Although,
we extend to look through dynamically generated managers on each
iteration instead of deferring until the final iteration.

mypy_django_plugin/errorcodes.py

@@ -0,0 +1,3 @@
+from mypy.errorcodes import ErrorCode
+
+MANAGER_UNTYPED = ErrorCode("django-manager", "Untyped manager disallowed", "Django")