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