Check for name conflicts on outer scoped tables

[gi0baro]

The relevant lines are https://github.com/web2py/pydal/blob/master/pydal/adapters/base.py#L612#L614

The original comment of @nextghost was

Fixing it would break backward compatibility with third-party code. outer_scoped is currently a list of internal table names. It would have to be changed into a dict of table objects, otherwise there is no way to check for conflicts (same name referencing different objects).