Ensure select_from_entity adapter is used in adjust_for_single_inheritance

Ensure select_from_entity adapter is used in adjust_for_single_inheritance

Fixed issue in single-inheritance loading where the use of an aliased
entity against a single-inheritance subclass in conjunction with the
:meth:`.Query.select_from` method would cause the SQL to be rendered with
the unaliased table mixed in to the query, causing a cartesian product.  In
particular this was affecting the new "selectin" loader when used against a
single-inheritance subclass.

Change-Id: Ic2cbe94a5269c101b1f98da9a466180dd4452783
Fixes: #4241