Alias newer "--library" to "--load-phutil-library" in legacy workflows

Summary: See PHI1772. This is a quick patch to make some debugging/testing tasks easier until remaining workflows modernize. See T13490.

Test Plan: Ran scripts/arcanist.php --library ..., got the same behavior as scripts/arcanist.php --load-phutil-library ....

Differential Revision: https://secure.phabricator.com/D21323