flake: define flake-parts options for colmena

This allows us to define options on the top level, so that we can write
modules to generate config for multiple hosts.

Idea from
1c51514586

flake.nix

@@ -70,7 +70,7 @@
 
   outputs = inputs@{ self, nixpkgs, nixpkgs-small, nixpkgs-unstable, flake-parts, colmena, ... }:
     flake-parts.lib.mkFlake { inherit inputs; } {
-      imports = [ ];
+      imports = [ ./flake ];
 
       systems = [ "x86_64-linux" "aarch64-linux" ];
 

flake/colmena.nix

@@ -0,0 +1,19 @@
+# https://git.lix.systems/the-distro/infra/src/commit/fbb23bf517206175764f154ddfd304b9ec501f87/colmena.nix
+{ lib, ... }: {
+  options.flake.colmena = lib.mkOption {
+    type = lib.types.submodule {
+      freeformType = lib.types.attrsOf (lib.mkOptionType {
+        name = "NixOS module";
+        description = "module containing NixOS options and/or config";
+        descriptionClass = "noun";
+        check = value: builtins.isAttrs value || builtins.isFunction value || builtins.isPath value;
+        merge = loc: defs: {
+          imports = map (def: def.value) defs;
+        };
+      });
+      options.meta = lib.mkOption {
+        type = lib.types.attrs;
+      };
+    };
+  };
+}

flake/default.nix

@@ -0,0 +1,5 @@
+{
+  imports = [
+    ./colmena.nix
+  ];
+}