Proposal: Deprecate (or rename) extsrc/

[Luke Mewburn <lukem%netbsd.org@localhost>]

I propose that we deprecate or remove the "extsrc/" tree,
as the name name-complete conflicts with "external/".

The "external/" tree was added in mid 2008.
A lot of the in-tree third-party source is now in this tree,
and developers need to traverse this tree a lot.

The "extsrc/" tree was added in late 2009.
Nothing in tree uses "extsrc/"; it's a placeholder for third-party
vendors to hook into the build for their own extensions.
There's no reason a vendor can't just integrate into the build
with local changes - that's what Wasabi Systems did in the early 2000s.

(Also, if I recall correctly, "extsrc/" without much consultation).

If some people require the "extsrc/" functionality in tree, then I propose:
1) A good case to retain the functionality should be made by them.
2) A better name than "extsrc/" should be chosen, that's not
   going to cause completion rage. Maybe "3rdparty"?


Thoughts?

Luke.