[Twisted-Python] Distributing apilinks.py

[Tom Prince]

Eventually, apilinks should just go away. I think I have convinced
Michael (pydoctor's maintainer) that pydoctor should just genereate an
objects.inv that sphinx can read. Then the regular :py:*: annoations can
be used, rather than the apilinks ones (which interpert <> backwards
anyway).

That doesn't address the issue of what to do in the mean time. Looking
at the code, it looks very similar to what the builtin extlinks
extension does; except that apilinks wants `path.to.object <title> and
extlinks wants `title <path.to.object>`. (But I guess this doesn't
address the issue of # in the links being put in the wrong place. But,
https://twistedmatrix.com/documents/current/api/twisted.web.iweb.IRequest.getSession.html
does exists (probably to make this kind of linking work.

  Tom