ENYO-113: Making "gathering" feature more robust

The first pass at this feature didn't work for libraries that
were included in package.js via relative paths (e.g. ../lib)
rather than using the $lib shorthand. We now normalize the
path to each CSS sheet during minification so that the path
looks the same regardless of whether $lib is used or not.

Enyo-DCO-1.1-Signed-Off-By: Gray Norton (gray.norton@lge.com)