[bug#46031] services: cuirass: Add "simple-cuirass-services".

[Mathieu Othacehe <othacehe@gnu.org>]

Hey,

> When a service extends a service type, an instance of that service type
> is automatically added if it’s missing (provided that service type has a
> default value).  This happens in ‘instantiate-missing-services’.

Oh! Didn't know about that one!

> So, if postgresql-role-service-type and postgresql-service-type have a
> default value, simple-cuirass-service could extend them both (possibly
> with a dummy value) and it would just work.

With 8163f74542300720f6ee5dc061b79ddf0c345bb8 and
108e2c6116f01c3b0a98498717d65a96c1857a51, I made sure that those
services are automatically instantiated when missing.

This means simple-cuirass-configuration->specs makes way more sense, as
you suggested. Took care of it with
bebcf97600b2fa65482ae8ee870800dafa34d3f8.

Now, as suggested by Jonathan on IRC yesterday, it would be nice to
be able to build only the packages of a given channel. For that, we
would need to be able to figure out which channel is providing a
package, with something like a package-channel field.

Do you think that would make sense?

Thanks,

Mathieu