_refs doesn't pick up references from Model Properties

Summary: If I have a Link Set defined in a Model Property, and then set Links in a Thing that uses that Property, _refs doesn't pick it up.

(Actual example is the Origin Restrictions Property in Spirit Island, but distilling the issue down:)

Create Link Set Prop, pointing to a few Things. Create Contained Model, and put Link Set Prop on it. Create My Property from Contained Model.

Now create Real Model, and Instance as a child of it. Set some links in Link Set Prop on Instance. If you look at the linked-to Things, _refs from them doesn't list Instance. Basically, _refs doesn't know how to go through a Model Property.

Note: the same bug is almost certainly true of _tagRefs as well, and should be fixed there too.