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 
After you went to the trouble of creating drivepath (which is non-empty only if it's a UNC drive) why not use it here and say ```if not (drive or drivepath)```?

Also, this logic is getting *very* convoluted (I appreciate that it wasn't a model of clarity before). Some named variables for things like ```is_unc``` or ```has_drive``` would probably be useful, as would some comments. I'm having trouble following it, I hate to think how it looks to someone not familiar with Windows and UNC paths!