Adjust natural path to relationship's base mapper for aliased class also

Adjust natural path to relationship's base mapper for aliased class also

Fixed regression in loader options introduced in 1.3.0b3 via :ticket:`4468`
where the ability to create a loader option using
:meth:`.PropComparator.of_type` targeting an aliased entity that is an
inheriting subclass of the entity which the preceding relationship refers
to would fail to produce a matching path.   See also :ticket:`5082` fixed
in this same release which involves a similar kind of issue.

Fixes: #5107
Change-Id: I5c6717b925060c3f8da42190d1f00d05248befd8
(cherry picked from commit c9bf876d93b9389d6d3b619565f6da166bab5ec2)