Restore detached object logic for dynamic, but warn

Restore detached object logic for dynamic, but warn

Fixed regression involving ``lazy='dynamic'`` loader in conjunction with a
detached object. The previous behavior was that the dynamic loader upon
calling methods like ``.all()`` returns empty lists for detached objects
without error, this has been restored; however a warning is now emitted as
this is not the correct result. Other dynamic loader scenarios correctly
raise ``DetachedInstanceError``.

Fixes: #6426
Change-Id: Id7ad204bef947491fa7e462c5acda2055fada910