Allow Javelin initBehavior to source alternative library behaviors

Summary: Ref T6467. Opens up initBehavior for non-phabricator sourced behaviors.

Test Plan: Confirmed no impact on unset (default 'phabricator' source name) calls to initBehavior

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: aklapper, Korvin, epriestley

Maniphest Tasks: T6467

Differential Revision: https://secure.phabricator.com/D10780