Add upstream metadata to cme to learn Config::Model ?

[Dominique Dumont <dod@debian.org>]

Hi

Cme and Config::Model currently ignore Debian upstream metadata (DEP-12).

I think adding this feature to cme is a good way to peek under cme's hood and 
learn how to describe a model for cme. This task does not require much 
knowledge of Perl

And this would also improve cme bus factor which is not great (i.e. 1, me, and 
I'm not getting any younger)

I've added more details and doc references in #966208 [1]. I'll be available 
to provide help if needed.

All the best

Dod

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=966208