use annotated entity when adding secondary

use annotated entity when adding secondary

Fixed regression in "dynamic" loader strategy where the
:meth:`_orm.Query.filter_by` method would not be given an appropriate
entity to filter from, in the case where a "secondary" table were present
in the relationship being queried and the mapping were against something
complex such as a "with polymorphic".

Fixes: #7868
Change-Id: I3b82eec6485c5a92b56a596da0cfb009e9e67883