Improve typing for unresolved managers (#1044)

* Improve typing for unresolved managers

This changes the logic when encountering an unresolvable manager class.
Instead of adding it as a `Manager` we create a subclass of `Manager`
that has `fallback_to_any=True` set. Similarly a `QuerySet` class is
created that also has fallbacks to `Any`. This allows calling custom
methods on the manager and querysets without getting type errors.

* Fix manager created and improve a test

* Fix row type of FallbackQuerySet

Because this inherits from _QuerySet, not QuerySet, it needs to have two
parameters

tests/typecheck/fields/test_related.yml

@@ -678,18 +678,24 @@
                     def custom(self) -> None:
                         pass
 
+                def TransactionManager():
+                    return BaseManager.from_queryset(TransactionQuerySet)()
+
                 class Transaction(models.Model):
-                    objects = BaseManager.from_queryset(TransactionQuerySet)
+                    objects = TransactionManager()
                     def test(self) -> None:
                         reveal_type(self.transactionlog_set)
                         # We use a fallback Any type:
+                        reveal_type(Transaction.objects)
                         reveal_type(Transaction.objects.custom())
 
                 class TransactionLog(models.Model):
                     transaction = models.ForeignKey(Transaction, on_delete=models.CASCADE)
     out: |
-        myapp/models:10: note: Revealed type is "django.db.models.manager.RelatedManager[myapp.models.TransactionLog]"
-        myapp/models:12: note: Revealed type is "Any"
+        myapp/models:11: error: Could not resolve manager type for "myapp.models.Transaction.objects"
+        myapp/models:13: note: Revealed type is "django.db.models.manager.RelatedManager[myapp.models.TransactionLog]"
+        myapp/models:15: note: Revealed type is "myapp.models.UnknownManager[myapp.models.Transaction]"
+        myapp/models:16: note: Revealed type is "Any"
 
 
 -   case: resolve_primary_keys_for_foreign_keys_with_abstract_self_model