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.

tests/typecheck/fields/test_related.yml

@@ -608,8 +608,14 @@
         reveal_type(Article().registered_by_user)  # N: Revealed type is "myapp.models.MyUser*"
 
         user = MyUser()
-        reveal_type(user.book_set) # N: Revealed type is "django.db.models.manager.RelatedManager[myapp.models.Book]"
-        reveal_type(user.article_set) # N: Revealed type is "django.db.models.manager.RelatedManager[myapp.models.Article]"
+        reveal_type(user.book_set) # N: Revealed type is "myapp.models.MyUser_Book_RelatedManager1"
+        reveal_type(user.article_set) # N: Revealed type is "myapp.models.MyUser_Article_RelatedManager1"
+        reveal_type(user.book_set.add)  # N: Revealed type is "def (*objs: Union[myapp.models.Book*, builtins.int], *, bulk: builtins.bool =)"
+        reveal_type(user.article_set.add)  # N: Revealed type is "def (*objs: Union[myapp.models.Article*, builtins.int], *, bulk: builtins.bool =)"
+        reveal_type(user.book_set.filter)  # N: Revealed type is "def (*args: Any, **kwargs: Any) -> myapp.models.LibraryEntityQuerySet[myapp.models.Book*]"
+        reveal_type(user.article_set.filter)  # N: Revealed type is "def (*args: Any, **kwargs: Any) -> myapp.models.LibraryEntityQuerySet[myapp.models.Article*]"
+        reveal_type(user.book_set.queryset_method())  # N: Revealed type is "builtins.int"
+        reveal_type(user.article_set.queryset_method())  # N: Revealed type is "builtins.int"
     installed_apps:
         - myapp
     files:
@@ -620,11 +626,13 @@
               class MyUser(models.Model):
                   pass
               class LibraryEntityQuerySet(models.QuerySet):
-                  pass
+                  def queryset_method(self) -> int:
+                      return 1
+              LibraryEntityManager = models.Manager.from_queryset(LibraryEntityQuerySet)
               class LibraryEntity(models.Model):
                   class Meta:
                       abstract = True
-                  objects = models.Manager.from_queryset(LibraryEntityQuerySet)()
+                  objects = LibraryEntityManager()
                   registered_by_user = models.ForeignKey(MyUser, on_delete=models.CASCADE)
               class Book(LibraryEntity):
                   pass