Update supera integration model in sbncode

[cerati]

Supera is currently integrated as git submodule in sbncode. This was meant to be a temporary solution. We should update the integration model, possibly taking advantage of the upcoming spack migration.

[cerati]

(based on conversations at the RM meeting today, with @ibsafa , @bear-is-asleep , @miquelnebot )

[ibsafa]

Tying this to the spack migration is a good idea. It would be good to get an update from the Supera/SuperaAtomic developers on the status and timeline so that we can be aware of upcoming changes on the release side and be sure we're setting realistic expectations.