Switch to using calculated absolution path for requires.

While there will be a small overhead added for this change it helps remove errors that can happen if `set_include_path` isn't set. In my case we didn't want to use `set_include_path` for libraries because it increase the number of `stat` calls done by PHP during the require of any other relative path.

This patch should be fully backwards compatible with people using `set_include_path`.