Improve support for with_polymorphic in mapper options

Improve support for with_polymorphic in mapper options

Improved the behavior of :func:`.orm.with_polymorphic` in conjunction with
loader options, in particular wildcard operations as well as
:func:`.orm.load_only`.  The polymorphic object will be more accurately
targeted so that column-level options on the entity will correctly take
effect.The issue is a continuation of the same kinds of things fixed in
:ticket:`4468`.

The path logic when using chained mapper options is improved
to be more accurate in terms of the entities being linked
in the path; when using :func:`.with_polymorphic`, mapper
options against this entity need to specify attributes
in terms of the with_polymorphic() object and not against the
base mappings.  New error conditions are raised which were previously
more than likely silenty failures.

Fixes: #4469
Change-Id: Ie8d802879663b4ff6f6ac1438c885c06d78ae2a0