Add fallback related manager in final iteration of AddRelatedManagers (#993)

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.

tests/typecheck/fields/test_related.yml

@@ -640,6 +640,42 @@
                   pass
 
 
+-   case: test_related_managers_when_manager_is_dynamically_generated_and_cannot_be_imported
+    main: |
+        from myapp import models
+    installed_apps:
+        - myapp
+    files:
+        -   path: myapp/__init__.py
+        -   path: myapp/models.py
+            content: |
+                from django.db import models
+
+                class User(models.Model):
+                    name = models.TextField()
+
+                def DynamicManager() -> models.Manager:
+                    class InnerManager(models.Manager):
+                        def some_method(self, arg: str) -> None:
+                            return None
+
+                    return InnerManager()
+
+                class Booking(models.Model):
+                    renter = models.ForeignKey(User, on_delete=models.PROTECT)
+                    owner = models.ForeignKey(User, on_delete=models.PROTECT, related_name='bookingowner_set')
+
+                    objects = DynamicManager()
+
+                def process_booking(user: User):
+                    reveal_type(user.bookingowner_set)
+                    reveal_type(user.booking_set)
+    out: |
+        myapp/models:3: error: Couldn't resolve related manager for relation 'booking' (from myapp.models.Booking.myapp.Booking.renter).
+        myapp/models:3: error: Couldn't resolve related manager for relation 'bookingowner_set' (from myapp.models.Booking.myapp.Booking.owner).
+        myapp/models:20: note: Revealed type is "django.db.models.manager.RelatedManager[myapp.models.Booking]"
+        myapp/models:21: note: Revealed type is "django.db.models.manager.RelatedManager[myapp.models.Booking]"
+
 -   case: foreign_key_relationship_for_models_with_custom_manager
     main: |
         from myapp.models import Transaction