remove setSource from default, start debug extension

Signed-off-by: buzzdecafe <m_hur@yahoo.com>

please review--i think it makes sense to use setSource only in debug mode, but i don't wanna merge without discussion/confirmation.

Also, this overriding thing may be a bit complex when we have `R` functions pointing at local variables e.g. `curry`; the build has to be able to override those internal functions somehow, or we have to change how they are used.