Make internal_backend plugins 1st class local pants plugins.

This moves wiring-up of the pants_internal backends to pants.ini
and make the (future) role of src/python/pants/bin:pants clear
and distinct from src/python/pants/bin:pants_local_binary.

It also fixes some broken docsite.json paths and adds some
TODOs to nuke 3rdparty/python/dev_only_requirements.txt.

https://rbcommons.com/s/twitter/r/1073/