Output metadata to Sphinx's object.inv

[adamtheturtle]

With the Python domain, one can use code like the following:

.. py:function:: spam(eggs)
                 ham(eggs)

   Spam or ham the foo.

With this, the py:function will be in the objects.inv file which is generated when the documentation is built:

~> python -m sphinx.ext.intersphinx ./build/html/objects.inv
py:function
	target                                   index.html#target
...

The same happens with C++ code wh

However, when the Sphinx Bazel domain is used, nothing is outputted to objects.inv.

When an object is referenced in objects.inv, there are multiple features available.

The one that I am interested in is to have the Bazel rules referenced in a Dash index using doc2dash.

[adamtheturtle]

I believe that this means implementing get_objects in BazelDomain.