Add flake-parts module

[MattSturgeon]

Description

Add a flake-parts module, output as flakeModules.home-manager and flakeModules.default.

The module defines options for flake.homeModules and flake.homeConfigurations, based on the respective nixos equivalents; flake.nixosModules and flake.nixosConfigurations.

This is mostly useful for people who want to define modules or configs in multiple flake modules. Without the options, they can still be defined, but the definition must be unique.

I've added some initial documentation, although I don't know how to build/browse the docs to check it renders how I expect.

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

[MattSturgeon]

@rycee sorry for the ping, but are you able to review?

[stale]

Thank you for your contribution! I marked this pull request as stale due to inactivity. Please read the relevant sections below before commenting.

If you are the original author of the PR

• GitHub sometimes doesn't notify people who commented / reviewed a PR previously when you (force) push commits. If you have addressed the reviews you can officially ask for a review from those who commented to you or anyone else.
• If it is unfinished but you plan to finish it, please mark it as a draft.
• If you don't expect to work on it any time soon, please consider closing it with a short comment encouraging someone else to pick up your work.
• To get things rolling again, rebase the PR against the target branch and address valid comments.

If you are not the original author of the PR

• If you want to pick up the work on this PR, please create a new PR and indicate that it supercedes and closes this PR.

[MattSturgeon]

I don't think there's any conflicts so I won't rebase just yet.

If someone is able to review I'm happy to address any feedback! 😁

[darren-recentive]

I don't think there's any conflicts so I won't rebase just yet.

If someone is able to review I'm happy to address any feedback! 😁

@rycee @onemoresuza pretty please 🙏

[khaneliman]

LGTM, thanks Matt!

[isabelroses]

Sorry for being a bit late but should the type not have been types.lazyAttrsOf types.deferredModule?

[MattSturgeon]

I wrote this a long time ago, referencing the flake.parts nixosModules module. So I would've used whatever type flake.parts was using at the time.

But yes, deferredModule is a more appropriate type. Good spot.