synonym fixes and enhancements

synonym fixes and enhancements

Fixed regression where an attribute that is mapped to a
:func:`_orm.synonym` could not be used in column loader options such as
:func:`_orm.load_only`.

Established support for :func:`_orm.synonym` in conjunction with hybrid
property, associationproxy, including that synonyms can be established
linking to these constructs which work fully. This is a behavior that was
semi-explicitly disallowed previously, however since it did not fail in
every scenario, explicit support for assoc proxy and hybrids has been
added.

Fixes: #6272
Fixes: #6267
Change-Id: Ie75bb3b535feeb6ccf3f6a391f21b69f241e625e