Add fallback related manager in final iteration of AddRelatedManagers

If a django model has a Manager class that cannot be resolved on runtime
(like django-money monkeypatches all default managers after models
already have been declared, and generates this manager as the return
value of a function, so it cannot be imported when we try to set up the
default related managers in AddDefaultManagerAttribute), we fallback to
a default related manager, so you at least get a base level of working
type checking.

mypy_django_plugin/errorcodes.py

@@ -1,3 +1,4 @@
 from mypy.errorcodes import ErrorCode
 
 MANAGER_UNTYPED = ErrorCode("django-manager", "Untyped manager disallowed", "Django")
+MANAGER_MISSING = ErrorCode("django-manager-missing", "Couldn't resolve manager for  model", "Django")

mypy_django_plugin/transformers/models.py

@@ -14,6 +14,7 @@
 from mypy.types import TypedDictType, TypeOfAny
 
 from mypy_django_plugin.django.context import DjangoContext
+from mypy_django_plugin.errorcodes import MANAGER_MISSING
 from mypy_django_plugin.lib import fullnames, helpers
 from mypy_django_plugin.lib.fullnames import ANNOTATIONS_FULLNAME, ANY_ATTR_ALLOWED_CLASS_FULLNAME, MODEL_CLASS_FULLNAME
 from mypy_django_plugin.lib.helpers import add_new_class_for_module
@@ -341,6 +342,7 @@ def run_with_model_cls(self, model_cls: Type[Model]) -> None:
                 continue
 
             if isinstance(relation, (ManyToOneRel, ManyToManyRel)):
+                related_manager_info = None
                 try:
                     related_manager_info = self.lookup_typeinfo_or_incomplete_defn_error(
                         fullnames.RELATED_MANAGER_CLASS
@@ -352,6 +354,15 @@ def run_with_model_cls(self, model_cls: Type[Model]) -> None:
                     if not self.api.final_iteration:
                         raise exc
                     else:
+                        if related_manager_info:
+                            self.add_new_node_to_model_class(
+                                attname, Instance(related_manager_info, [Instance(related_model_info, [])])
+                            )
+                            self.ctx.api.fail(
+                                f"Couldn't resolve related manager for {model_cls}",
+                                self.ctx.cls,
+                                code=MANAGER_MISSING,
+                            )
                         continue
 
                 # Check if the related model has a related manager subclassed from the default manager