Adjust use_mapper_path rule for poly subclasses

Adjust use_mapper_path rule for poly subclasses

We must change the approach from 2734439 as the information
loss is breaking subquery eager loading.

Move the adjustment into a deeper set of logic inside
of path_regsitry.  We can distinguish between a path
that will "naturally" build from an aliased entity
at the base, vs. one that will "naturally" build
on all raw mappers, based on if when we observe that
we are being given a with_polymorphic(), if the existing
parent path is already in progress or not.

In general, we prefer paths to have as much of the original
information as possible, and the "natural path" is supposed
to be where the loader lookup stuff happens.

Fixes: #5082
Change-Id: I3c0ee72993bae8a6f067bdef3dc9a57d83f64950