Early-assign Base.registry to a private name

Early-assign Base.registry to a private name

Fixed bug where user-mapped classes that contained an attribute named
"registry" would cause conflicts with the new registry-based mapping system
when using :class:`.DeclarativeMeta`. While the attribute remains
something that can be set explicitly on a declarative base to be
consumed by the metaclass, once located it is placed under a private
class variable so it does not conflict with future subclasses that use
the same name for other purposes.

Fixes: #6054
Change-Id: I1f2e04b0d74c493e7e90eadead4e861d8960a794