[plt-scheme] proposal for indicating planet package version numbers

Thanks for the discussion everyone.

I've thought about it some more and I'm now with the heretic, both in
spirit and with his suggestion for how to change it (except I went
with "External version" vs "PLaneT version").

There are more sophisticated things one could do (eg, Eli privately
suggested having a separate mapping from package versions to planet
versions and not even having the planet versions visible to the users
(unless they were synonymous with the package versions of course),
which seems like a good idea, but I think the simpler change is
probably better, at least for now.)

So, I've made the changes in my development version of PLaneT and I'll
release it at some point (I'm waiting on some other folks for other
things that will also change, so I can't release it right away.).

Thanks, Neil!

Robby