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 
The UNC path comes directly from the find_links parameter, thus, it can be a path. See index.py around line 360:
   
     # We trust every directly linked archive in find_links
     [Link(url, '-f', trusted=True) for url in self.find_links]

If you find no objection, I will change this to convert the UNC path to an url inside the Link constructor (in case it is a path).