Support filter_by() from columns, functions, Core SQL

Support filter_by() from columns, functions, Core SQL

Fixed regression where :meth:`_orm.Query.filter_by` would not work if the
lead entity were a SQL function or other expression derived from the
primary entity in question, rather than a simple entity or column of that
entity. Additionally, improved the behavior of
:meth:`_sql.Select.filter_by` overall to work with column expressions even
in a non-ORM context.

Fixes: #6414
Change-Id: I316b5bf98293bec1ede08787f6181dd14be85419