Good point, David.  Jeremy, Trent, you're the only other Windows buildbot operators as far as I know; feel free to clean up the old externals locations as you like.

Also, sorry to make it a bit hairier to operate, but I think this is a big enough improvement for day-to-day development that I definitely don't want to change back.  For the record, it is fairly easy to just move out the externals folder, do whatever you need to with the repository, and move it back in without having to rebuild OpenSSL or Tcl/Tk/Tix.