If a django model has a Manager class that cannot be resolved statically
(if it is generated in a way where we cannot import it, like `objects =
my_manager_factory()`), we fallback to the default related manager, so
you at least get a base level of working type checking.
* 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.
