Extending Julia domain from Python domain is causing indexing issues

[agjohnson]

Moving readthedocs/readthedocs.org#1382 here, as we've finally discovered the issue behind why search results sometimes mention Python objects.

The Sphinx Python domain uses hardcoded prefixes for building indexes, and since the Python domain is also loaded as part of the Julia domain subclassing, all objects are indexed under the Python and Julia domains. The domain should either be patched to work around the areas where this is hardcoded, or a proper domain should be written.