flake: expose {darwin,nixos}Modules automatically

[zowoq]

@zimbatm Do you mind if we reuse this code from blueprint?

[zimbatm]

I don't mind, but I'm curious: how would blueprint have to change to be useful for the infra repo?

[zowoq]

I don't think there is anything specific that would need to change in blueprint, I think what we'd need to consider is do we want to opt out of the flake-parts/flakeModule ecosystem, e.g. effects are useable without flake-parts but the integration is nicer.

[zowoq]

I did a first pass on blueprint in #1563. I'll probably continue working on it occasionally but I'd likely postpone any decision on switching until after we've using effects on buildbot.