Add DeclarativeMeta to globals

Add DeclarativeMeta to globals

Fixed issue in mypy plugin where newly added support for
:func:`_orm.as_declarative` needed to more fully add the
``DeclarativeMeta`` class to the mypy interpreter's state so that it does
not result in a name not found error; additionally improves how global
names are setup for the plugin including the ``Mapped`` name.

Introduces directory oriented testing as well, where a full
set of files will be copied, mypy runs, then zero or more patches
are applied and mypy is run again, to fully test incremental
behaviors.

Fixes: sqlalchemy/sqlalchemy2-stubs/#14
Change-Id: Ide785c07e19ba0694e8cf6f91560094ecb182016