Check explicitly for mapped class as secondary

Check explicitly for mapped class as secondary

Added a comprehensive check and an informative error message for the case
where a mapped class, or a string mapped class name, is passed to
:paramref:`_orm.relationship.secondary`.  This is an extremely common error
which warrants a clear message.

Additionally, added a new rule to the class registry resolution such that
with regards to the :paramref:`_orm.relationship.secondary` parameter, if a
mapped class and its table are of the identical string name, the
:class:`.Table` will be favored when resolving this parameter.   In all
other cases, the class continues to be favored if a class and table
share the identical name.

Fixes: #5774
Change-Id: I65069d79c1c3897fbd1081a8e1edf3b63b497223