Improve conformance with Debian Ada policy

Nicolas Boulenguez:
> At first glance, a simple NMU should be sufficient for plplot. The
> part building an Ada library is already not conformant with the Ada
> policy, so an NMU would not make the situation worst.
> Also, the package is involved in the python3.7 migration.
> I suggest to wait after these transitions to make the package
> conformant with the Ada policy (this will include a passage through
> NEW).

Hello.

The attached patch improves the conformance of the package with the
policy for Ada in Debian.

Right now, I miss time to even test it,
but the log for #821278 requires help with the Ada parts,
so it may at least be useful as a first draft.

You should decide if it is better to apply it now (to complete the
transition quickly) or after the transitions (not renaming the Ada
-dev package may break existing user builds as described in the policy
for Ada).

Feel free to contact me for any help with the Ada binding.