Generalize DescriptorProps.uses_objects

Generalize DescriptorProps.uses_objects

Previously, uses_objects was specific to the SynonymAttribute;
generalize it so that it defaults to False for other DescriptorProps.
Immediate fix is against CompositeProperty.

Fixed regression introduced in 1.3.0 related to the association proxy
refactor in :ticket:`4351` that prevented :func:`.composite` attributes
from working in terms of an association proxy that references them.

Add test coverage for association proxies that refer to Composite
attributes as endpoints.

Fixes: #5000
Change-Id: Iea6fb1bd3314d861a9bc22491b0ae1e6c5e6340d