[feature request] mark own modules as not imported by default

I would like to be able to mark certain modules defined in my flake as not imported to all systems by default.

Currently, I have a module that configures a buildbot-nix worker. However, this means that I also have to add buildbot-nix.nixosModules.buildbot-worker to systems.modules.nixos so that all systems pick it up. This increases evaluation time and is also a lot "dirtier" in terms of what I'm importing, since most machines will not have the buildbot worker on them. Therefore, if I could mark a module as non-importable somehow, I could make it so that only machines that will run a worker will import this, and by extension, also the buildbot-worker module from buildbot-nix. Is this possible at all with snowfall?

Start with modules/nixos/services/buildbot/default.nix and the systems.modules.nixos configuration described in the issue. Investigate whether snowfall provides a way to exclude a module from default imports; done means a working opt-out that limits the buildbot-worker import to selected systems and avoids importing it everywhere.