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 
Comment (at least) badly needed here. Refactoring it is probably better. I *think* that url only starts with ```'//'``` if it's a UNC path, but that's mainly guessing the intent of the code and working back to what it's saying.