feat(class): Classes for flake exposed system outputs

[vic]

Changeset Summary

Moved forward into nix/lib

forward proved to be a core combinator, moving it to lib allows to be used outside of Den's NixOS framework.

Forward based Ouputs

Now all nixosConfigurations,homeConfigurations,etc are now generated via forward itself, accumulating into a flake-class module

Flake System Outputs

Den now supports exposing to flake outputs via aspects.

The following classes were added (each named after the corresponding flake output attribute): packages, apps, checks, legacyPackages, devShells. All these are per-system outputs.

Usage:

See templates/ci/modules/features/forward-flake-level.nix for all tests

den.aspects.foo = {
  packages = { pkgs, ... }: { inherit hello; };
  devShells = { pkgs, ... }: {
    default = pkgs.mkShell { buildInputs = [ pkgs.hello ]; };
  };
}

# Enable flake output attributes
imports = with inputs.den.flakeOutputs; [ packages devShells ];

# Include aspects that produce per system flake outputs:
den.ctx.flake-system.includes = [ den.aspects.foo ];

You can also make all your hosts produce flake-level outputs by simply
creating a context transition { system } -> { host }:

den.ctx.flake-system.into.host = { system }:
  map (host: { inherit host; }) (lib.attrValues den.hosts.${system});

flakeOutputs.* modules. See #317

Den now provides the following modules under inputs.den.flakeOutputs.*

• flake: creates a generic options.flake top-level option. Used only when no inputs.flake-parts is available to avoid collision with flake-parts.
• nixosConfigurations: creates options.flake.nixosConfigurations
• darwinConfigurations
• homeConfigurations
• packages: creates per-system options.flake.packages
• apps
• checks
• devShells
• legacyPackages

You will only need to import these on your flake (like we did in the previous code example) if you want to enable those flake outputs being written by Den.

den.systems option

This option is used by den.ctx.system to know what systems to produce outputs for.
It default to config.systems (flake-parts top-level system if it exists), or systems of den.hosts + den.hosmes, or if those were empty: lib.systems.flakeExposed

nvf-standalone using packages output

Our nvf example was the first iteration on producing packages from Den aspects, it has been updated to use den.ctx.flake-packages now.