Is there any simple way to distinguish between "home grown" sources in
our source tree and ones which are imported from external maintainers?

Obviously, everything in gnusrc is external, but things like nvi and
sendmail are in the src tarball even though they come from the Great
Outside.

Ideally, what I'd like to do is set things up so that only sources which
originate from NetBSD get supped from there, and sources that come from
elsewhere get supped from those elsewheres (or else downloaded periodically
those places don't do sup).

--Mirian