cache vs don't cache absolute filepaths

I wrote this: https://github.com/Shyp/cache-require-paths/commit/f21238ca914a96b6b53f7945d33b48a9b1ae4e1c

then realized it's not the right thing really... we don't care about absolute vs. relative, so much as we care about whether the path references something on the filesystem, or a module name..

For what it's worth I didn't see any measurable perf difference with this patch, but I wasn't rigorous about testing it.