When using an alternate path, resolve the path before using it.

There is an edge case where if your src file is outside of your `gulpfile.js` directory, your alternate path may not work under certain circumstances (such as if the alt path is up any directories).

To create a test for this, I'd have to have files outside of the repo, or change where the `gulpfile.js` is, so it's not a trivial thing.

This change does not break any current changes and fixes the described edge case issue I was having.