a few cross-platform / windows bugs

While working on a build process for a large Jenkins cluster I stumbled across a few bugs on the Windows build nodes.

With these fixed, everything is working great, even on Windows Server 2012.

Fix 1
> File URIs supplied as CPAN mirrors would not work on Windows because of the way it normalised the    path for use with File::Copy

Fix 2
> maybe_abs was creating bad paths on Windows, which in turn broke a lot of stuff, and most importantly, ended up passing strange installation locations to things