Add support for publishing_group per ADR0037

[Gchbg]

Release note:

Add support for publishing_group

[nkraetzschmar]

I'm not sure this addition of default values to the requirements_keys array is the cleanest approach here. Maybe if we now in addition to defining what the available keys are we also want to dynamically set their default values, it might be a good idea to rethink the approach that the set of possible requirement keys is defined by features.

Instead a cleaner option might be to introduce one top level requirements.def file (next to the get_repo and similar files) that both defines the keys of all known requirements as well as their default values. The per feature requirements.mod files then simply operate on these variables.

This would also let us revert #151 which was only added to enable discovery of requirements in features that are not included.