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