Revert "Simplify the use of phutil_get_library_root"

Summary:
This reverts commit 67a17d0efb025665237d0d174d55d1887f9a6064.

This doesn't actually work because the call to phutil_get_current_library_name() always occurs within libphutil.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13187