[Dune] Improve module handling by CMake

[Christian Engwer]

> - Should we create some kind of working group to go into the details of the cmake system? How could this be organized? I think, sending messages here to the mailinglist is a good starting point, but makes productive discussion complicated. Maybe some topic specific Matrix/Slack/Mattermost/... chat room? Or a GitLab Issue in the dune-common repo?

I think gitlab ist the way to go, because it also documents the
decision process. If people step in later it is important that they
can catch up on the arguments.

Best
Christian