Re: V4 Pre-PEP: transition to release-file hosting on PYPI

Do we even need the internal/external rel info?  I was planning to
just use the URL hostname.

i.e., are there any use cases for designating an externally-hosted
file internal, or an internally-hosted file external?  If not, it
seems the rel="" is redundant.

It's also more work to implement, vs. just defaulting --allow-hosts to
be the --index-url host; a strategy ISTM pip could also use, since it
has the same two options available.

Also, if we're not doing homepage/download crawling any more, I was
hoping we could just drop the code that 'parses' rel="" links in the
first place, as it's an awkward ugly hack.  ;-)