allowing coexistence for -dev packages

[Nicolas Boulenguez <nicolas.boulenguez@free.fr>]

Unless I miss something in the current policy for Ada packages,

http://people.debian.org/~lbrenta/debian-ada-policy.html

the scenario "coexistence allowed" for library foo is the same than
the scenario "coexistence not allowed" for library foo4, except that:
- the dev package is named libfoo4-dev, not libfoo4-V-dev
- the lib package is named libfoo4, not libfoo4-N
- there is neither soversion nor aliversion for foo4.
  (if anything changes, a new library named foo5 is born)

Why not drop the "coexistence allowed" scenario, and consider that
gtkada2, gtkada3, gnat4.6 and gnat4.8 are distinct library names, each
one of them following the "coexistence not allowed" scenario?

Bonus: each library would now have its own ali and so version. For
example, libgtkada2-24.1-dev would Break/Replace libgtkada2-23.7-dev,
but could coexist with libgtkada3-3.1-dev and later with
libgtkada3-3.2-dev.