trust user PK argument as given; don't reduce

trust user PK argument as given; don't reduce

Fixed issue where the :class:`_orm.Mapper` would reduce a user-defined
:paramref:`_orm.Mapper.primary_key` argument too aggressively, in the case
of mapping to a ``UNION`` where for some of the SELECT entries, two columns
are essentially equivalent, but in another, they are not, such as in a
recursive CTE. The logic here has been changed to accept a given
user-defined PK as given, where columns will be related to the mapped
selectable but no longer "reduced" as this heuristic can't accommodate for
all situations.

Fixes: #7842
Change-Id: Ie46f0a3d42cae0501641fa213da0a9d5ca26c3ad