PJSIP: Enforce module load dependencies

PJSIP: Enforce module load dependencies

This enforces that res_pjsip, res_pjsip_session, and res_pjsip_pubsub
have loaded properly before attempting to load any modules that depend
on them since the module loader system is not currently capable of
resolving module dependencies on its own.

ASTERISK-24312 #close
Reported by: Dafi Ni
Review: https://reviewboard.asterisk.org/r/4062/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@425690 65c4cc65-6c06-0410-ace0-fbb531ad65f3