Avoid resolving path in celerity resource map

Summary:
HPHP doesn't like resolved symlinks.

Also I like this code better.

Test Plan: Used and not used custom Celerity map.

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2694