Fix handling of file paths in module dependencies

[ghost]

This fixes the handling of file paths in module dependencies as specified with --requirements, which previously led to them simply being ignored (and hence breaking any build that would try to use this).

[ghost]

This is by the way rather relevant to work around #1488 (because you can copy your project folder and specify that as a dep to make sure it's setup.py installed) so it's not quite as theoretical as it may sound like

[inclement]

Seems fine, although I'll note that this it not so much fixing the handling of file paths as adding support for modules supplied as file paths. I don't think this has ever been intended to work. It's interesting that it takes so little work to get it going.

[ghost]

@inclement I can't rule out other broken corner cases, but it worked fine in my tests. Please note this doesn't fix recipe's not handling them though, it just fixes them not working properly for --requirements

[ghost]

Updated the code a little such that it leaves URLs alone. Other than that, it works for me, and I encourage others to give it a try. I am depending on an increasing pile of unmerged changes to get my project to build, so it'd be neat to see particularly this file path support go in some time soon