Sphinx smart resolve Bases from intersphinx for affiliated packages

Some of the ``Bases`` links are broken and seems unfixable in affiliated packages.  Examples are in this issue of photutils: https://github.com/astropy/photutils/issues/152

(e.g. as the ``Bases`` of a class, ``astropy.modeling.core.Fittable2DModel`` isn't resolved to ``astropy.modeling.Fittable2DModel``)

This PR fixes this issue in photutils. I've also tested it with the core astropy, and it didn't broke anything there.

cc: @astrofrog 
Thanks for spotting this. It would have taken quite a while to figure out what went wrong it if by chance it didn't work as-is. :smile: