export module as system-independent flake output

[AluisioASG]

The example in the README uses agenix.nixosModules.age as the NixOS module, but that causes an "attribute 'age' missing" error for me. I've had to use agenix.nixosModules.x86_64-linux.age instead. nix flake show sees no module in there either:

nix flake show 'github:ryantm/agenix'
github:ryantm/agenix/8af97149b28adaf28699af923c35633310d52edf
[…]
├───nixosModules
│   ├───aarch64-linux: NixOS module
│   ├───i686-linux: NixOS module
│   ├───x86_64-darwin: NixOS module
│   └───x86_64-linux: NixOS module
[…]

This is because flake-utils doesn't try to distinguish outputs with ${system} keys from those without.

In my own flakes I've been using a pattern inspired by this idea for system-dependent and system-independent outputs, and it's the same pattern I'm using in this PR.

Because someone may already be using the current paths, I've kept nixosModules as is, and added a singular nixosModule output, which isn't recognized by nix flake show but is documented in the wiki. Alternatively I can change nixosModules to match the documentation,

I'm marking this as WIP because the README needs to be updated based on the previous paragraph.

[ryantm]

I'm okay breaking a handful of people now to get the output to be standard.

[AluisioASG]

Done. It should match the README now.

[ryantm]

Thanks.

When I run nix flake show on the repo after checking out your branch, I get an error message:

$ nix --version
nix (Nix) 2.4pre20201102_550e11f
$ nix flake show
warning: unknown setting 'extra-sandbox-paths'
git+file:///home/ryantm/p/agenix?ref=flake-nixos-module&rev=c1cbfe75b0e2b26fa0918c5634d13c8364d04599
├───defaultPackage
│   ├───aarch64-linux: package 'agenix'
│   ├───i686-linux: package 'agenix'
│   ├───x86_64-darwin: package 'agenix'
│   └───x86_64-linux: package 'agenix'
├───nixosModules
│   └───age: NixOS module
└───packages
    ├───aarch64-linux
    │   ├───agenix: package 'agenix'
error: --- Error ---------------------------------------------------------------------------------------------------------------------------- nix
expected a derivation

Do you get this error? I'll try upgrading my version of Nix.

[ryantm]

Still happens on a later version

$ nix --version
nix (Nix) 2.4pre20201201_5a6ddb3

[ryantm@home1:~/p/agenix]$ nix flake show --show-trace
git+file:///home/ryantm/p/agenix?ref=flake-nixos-module&rev=c1cbfe75b0e2b26fa0918c5634d13c8364d04599
├───defaultPackage
│   ├───aarch64-linux: package 'agenix'
│   ├───i686-linux: package 'agenix'
│   ├───x86_64-darwin: package 'agenix'
│   └───x86_64-linux: package 'agenix'
├───nixosModules
│   └───age: NixOS module
└───packages
    ├───aarch64-linux
    │   ├───agenix: package 'agenix'
error: --- Error --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- nix
expected a derivation

[AluisioASG]

Yeah, it already happened before this PR. I elided it from my original post because it wasn't relevant to the module thing.

Replicating the packages line from flake.nix:

nix-repl> (import <nixpkgs> { system = "x86_64-linux"; }).callPackage ./default.nix {}
{ agenix = «derivation /nix/store/261y133fid9pmv7q6rnkjnznss503wkk-agenix.drv»; override = { ... }; overrideDerivation = «lambda @ /nix/store/2vjfc00qrhm648x2rzb3gfqxammif8mp-source/lib/customisation.nix:84:32»; }

I suspect Nix is choking on override/overrideDerivation.

[AluisioASG]

(As far I can tell it works fine, just not when doing nix flake show or nix flake check.)

[ryantm]

Okay, I'm glad it isn't just me. Merged!