path: make basename() on Windows case-insensitive
path.relative()
correctly treats Windows as case-insensitive.
path.basename()
should do the same when supplied with an extension.
CI: https://jenkins-iojs.nodesource.com/job/iojs+any-pr+multi/117/
(Came around to this because I'm going through "TODO" comments in the code. This one seemed sensible to me. But if the consensus is that this is a bad idea, then I can go back and instead just remove the TODO comment and be done with it.)