Improve package and base OS version support
Currently the OBS package build checks triggered by pull requests do not work anymore. When we introduce new dependencies or require a new version of our dependencies we should be careful that the base operating systems still provide all what is needed.
- Update contribution hints or review checklists, e.g. to look out for package builds, make sure packages and updated packages are submitted to openSUSE Factory and whatever current Leap version is in development
- Evaluate if we can enable tests for other package builds in the spec file, than just openSUSE_Factory, e.g. also openSUSE_Tumbleweed and openSUSE_Leap_15.1