Use Filesystem::pathExists
Summary: Use the Filesystem::pathExists method in the phutil_get_library_root_for_path function.
Test Plan: arc unit
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D11327