This moves a bunch of stuff out of flake.nix into the modules they
belong to. This removes complexity from flake.nix and gives the project
a more organised structure.
Sadly, it is not possible to import modules from a flake outside of
flake.nix, since that leads to an infinite recursion (`config` has to be
evaluated before `config._modules.args.inputs` is available but `config`
depends on an import from `config._modules.args.inputs`). Therefore, the
`extraModules` argument in `machines/default.nix` has to be used for
that (it now has access to all flake inputs).
Simon Bruder