[ty] Include definition site for "members of" API
In the course of writing the "add an import" implementation,
I realized that we needed to know which symbols were in scope
and how they were defined. This was necessary to be able to
determine how to add a new import in a way that (minimally)
does not conflict with existing symbols.
I'm not sure that this is fully correct (especially for
symbol bindings) and it's unclear to me in which cases a
definition site will be missing. But this seems to work for
some of the basic cases that I tried.
