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