consider _ClassStrategyLoad as part of endpoint path

consider _ClassStrategyLoad as part of endpoint path

Added new capability to the :func:`_orm.selectin_polymorphic` loader option
which allows other loader options to be bundled as siblings, referring to
one of its subclasses, within the sub-options of parent loader option.
Previously, this pattern was only supported if the
:func:`_orm.selectin_polymorphic` were at the top level of the options for
the query.   See new documentation section for example.

As part of this change, improved the behavior of the
:meth:`_orm.Load.selectin_polymorphic` method / loader strategy so that the
subclass load does not load most already-loaded columns from the parent
table, when the option is used against a class that is already being
relationship-loaded.  Previously, the logic to load only the subclass
columns worked only for a top level class load.

Fixes: #10348
Change-Id: I13a8cad5484e97b2ab51402f100193cb054e2155