use correct entity in path for aliased class relationship

use correct entity in path for aliased class relationship

Fixed bug in "relationship to aliased class" feature introduced at
:ref:`relationship_aliased_class` where it was not possible to create a
loader strategy option targeting an attribute on the target using the
:func:`_orm.aliased` construct directly in a second loader option, such as
``selectinload(A.aliased_bs).joinedload(aliased_b.cs)``, without explicitly
qualifying using :meth:`_orm.PropComparator.of_type` on the preceding
element of the path. Additionally, targeting the non-aliased class directly
would be accepted (inappropriately), but would silently fail, such as
``selectinload(A.aliased_bs).joinedload(B.cs)``; this now raises an error
referring to the typing mismatch.

Fixes: #7224
Change-Id: I40857c7275667dcb64f1d1fd0c8072e48758e678