Fixed relative paths

[rtsao]

In the https://github.com/css-modules/browserify-demo repo, all the composes are commented out, for example:

StyleVariantA.css

.root {
  /*composes: box from "../../../shared/styles/layout.css";*/
  border-color: red;
}

.text {
  /*composes: heading from "../../../shared/styles/typography.css";*/
  color: red;
}

Uncommenting these lines results in ENOENT on build.

This PR fixes the resolution of these relative paths if we uncomment them.

[markdalgleish]

Could you squash these into a single commit? No need for us to merge in a broken commit :)

[rtsao]

Oops forgot to do that, I've gotten too used to Phabricator and having Arcanist do that automatically. I've just fixed that.

[geelen]

Looks good to me!

[markdalgleish]

Nice work—thanks so much for fixing this @rtsao, this issue was next on my list 😄