Set custom queryset methods as manager attrs instead of method copies (#820)

Instead of copying methods over from a QuerySet passed to a basemanager
when invoking '<BaseManager>.from_queryset', any QuerySet methods are
declared as attributes on the manager.

This allows us to properly lookup any QuerySet method types via a
'get_attribute_hook' and will thus remove disorienting phantom errors
occuring from mypy trying to resolve types only existing in the module
where the _original_ (and real) queryset method was declared.

tests/typecheck/fields/test_related.yml

@@ -684,6 +684,7 @@
         reveal_type(User().orders)  # N: Revealed type is "myapp.models.User_Order_RelatedManager1"
         reveal_type(User().orders.get())  # N: Revealed type is "myapp.models.Order*"
         reveal_type(User().orders.manager_method())  # N: Revealed type is "builtins.int"
+        reveal_type(Product.objects.queryset_method())  # N: Revealed type is "builtins.int"
         reveal_type(Order().products)  # N: Revealed type is "myapp.models.Order_Product_RelatedManager1"
         reveal_type(Order().products.get())  # N: Revealed type is "myapp.models.Product*"
         reveal_type(Order().products.queryset_method())  # N: Revealed type is "builtins.int"