Move verilog-mode and vhdl-mode to elpa.gnu.org

[Glenn Morris <rgm@gnu.org>]

Hello verilog and vhdl mode maintainers,

Recently I've been trying to keep the version of these modes in Emacs
up-to-date with your versions. I'd prefer not to keep doing this (I don't
actually use either mode).

I think a much better solution for everybody would be to move both these
packages to elpa.gnu.org. If you are not familiar with this, it is a
gnu.org package repository for Emacs lisp files. Other people on this
list know much more about it than I do, and can hopefully ask any
questions you might have.

One of the main advantages is that the files can be updated on your own
schedule, independent of the Emacs release cycle. This should make the
statement on http://www.veripool.org/projects/verilog-mode/wiki/Installing
about the Emacs version lagging the external version "by months to
years" a thing of the past! :)

Ideally, you would update the elpa.gnu.org versions yourselves.
It's essentially just a git repository. I believe there is some way to
add external branches; again, others can say more about this and the
best way to do it. I see verilog mode already use git. vhdl mode seems
to have no public VCS, which makes merging less than fun. Perhaps
elpa.gnu.org could be that VCS.

Anyway, regardless of the details, I think it's The Right Thing to Do.