Add support for extraModules input

[oliverwiegers]

NixOS offers the possibility to pass a list of extraModules to lib.nixosSystem which later gets merged with the modules list and baseModules.
This offers the possibility to read additional modules from environment variables, or a custom module-list.nix.

NixOS implementation is
here.

For nix-darwin an implementation was started in #592. I restarted the implementation and removed the usage of NIXOS_EXTRA_MODULE_PATH env variable because NixOS deprecated this one.

This is my first PR. Anything missing?

[oliverwiegers]

@emilazy @Enzime I can't request a review. Is just opening the PR and waiting the way to go?

Let me know if you need anything from me :)

[emilazy]

I’d be okay doing this just on general NixOS interface compatibility grounds, but I’m curious what value it adds for you without the environment variable, since there’s no difference between setting extraModules and just adding things to modules?

[oliverwiegers]

@emilazy you're right it does not add any additional functionality without the environment variable. I would be happy to keep it, but I wanted to make it as close to the NixOS implementation as possible.

My main goal (should have stated this earlier) is to provide a way to separate different modules when invoking darwinSystem to make the function call more readable.
As an example: By having extraModules as an input I can put my configuration.nix and third party modules in the modules input and my custom modules in extraModules.

In short: Make ones personal config easier to read.

Does that make sense?

P.S. Using the variable would still be possible in a somewhat readable way:

lib.darwinSystem {
  ...
  modules = [configuration.nix];
  extraModules = [
    { imports = [ (builtins.getEnv "NIXOS_EXTRA_MODULE_PATH") ]; }
  ];
  ...
}

Which I think is more readable that putting both in modules

[oliverwiegers]

@emilazy if this isn't something you would like to add you can close the PR 😊