Add a phutil_get_current_library_name function
Summary: See https://secure.phabricator.com/D9827?id=23585#inline-37952
Test Plan: Called the function from within arcanist and verified that 'arcanist' was returned.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9828