Allow lookups on .annotate fields

[sterliakov]

Allow lookups on fields coming from .annotate()

Django does not object when fields introduced via QuerySet.annotate are used in .filter with lookup chains:

from django.db import models

class Blog(models.Model): pass

Blog.objects.annotate(id2=models.F('id')).filter(id2__gt=10)

mypy, however, rejects this with a really cryptic error message:

error: Cannot resolve keyword 'id2' into field. Choices are: id, id2  [misc]

Since now all .annotate fields are resolved as plain Any, I think it's fair to allow chaining any lookups with them. This PR does exactly that: when a id2__something lookup is encountered and id2 is not a model field, proceed with checking that id2 is .annotate field and accept if so, ignoring the chained lookups.

[flaeppe]

Nice, just one fix with the tests

[sterliakov]

Tangentially related to #2306 and #2302? This should get rid of lookup problem there, but still doesn't support alias.

I'll grab .alias() as well, it also causes problems in my codebase:)

[flaeppe]

I think we need a third type argument on QuerySet to resolve alias?

Or at least that might be one alternative way