Add support for Windows UNC paths specified as --find-links

This pull request allows specifying windows UNC paths to the --find-links option.

At our workplace, we have the need to host our python packages in-house on a network share.

Pip did not allow installing from this share. This patch provides the use case.

    pip install --no-index -f \\unc\as\path package
    pip install --no-index -f file://///unc/as/path package 

Please advise on the quality or approach.

Fixes #1846 
Hmm, OK. Looks like this line is unrelated to the UNC changes, then, rather it's simply  an existing test that doesn't work properly on Windows. Sadly, it's not the only one...

But pathname2url forces the drive to uppercase (nturl2path.py line 60 in Python 3.4) so I'd treat that as standard and say path_to_url is wrong here to leave it as lowercase. I've no idea if there's a standard definition of how file URLs for case insensitive systems should be canonicalised, but I'd treat the stdlib as definitive in the absence of anything better.

If you're switching to use pathname2url, this might just go away. Otherwise, adding a comment that this is a hack to get the test to work on Windows (and why it's needed) would be enough for now.

As you can see, this stuff is a huge can of worms :-(