Remove some unnecessary defer rounds

[flaeppe]

Change so that we don't trigger unnecessary defer rounds during manager creation via the from_queryset method call.

Refs:

• Improve typing for unresolved managers #1044
• Could not resolve manager type for "core.models.FooModel.objects" [django-manager-missing] #1023

[flaeppe]

I'm going to argue that the changes here improves correctness. Since the manager in the test comes from a function that specifies a manager as return type:

def LocalManager() -> models.Manager:
    """
    Returns a manager instance of an inlined manager type that can't
    be resolved.
    """
    class InnerManager(models.Manager): ...
    return InnerManager()

This (now) assigns a type models.Manager[Any], the return type of the function. I would say that the previous behaviour did too much, since it tried to express that assigned an invalid manager. But to know that it had to draw conclusions from the function body and thus "override" the return type.

The plugin should now only consider calls of the format: objects = <Manager>.from_queryset(<QuerySet>)() or objects = <QuerySet>.as_manager() in this case

[flaeppe]

To clarify a little bit more; previously the plugin could tell mypy to defer on e.g. a case like this:

def not_a_manager() -> Any: ...

class SomeModel(models.Model):
    objects = not_a_manager()

Deferring isn't gonna make it a more of a manager. The changed code looks for a more narrow/specific case that we want and can handle that comes from from_queryset. For any other situation we now just continue without deferring.

[flaeppe]

@sobolevn, @intgr would any of you guys mind checking this out? I'm suspecting it could improve a couple of errors in #1023 (though I have no proof that it will)