flake: add hosts

This will allow other modules to cross-reference which hosts exist on
which system.

My main use-case is to automatically declare home-manager configuration
for the home configuration of NixOS hosts.

I also include Darwin in case I ever want to use that in the future,
though that is unlikely for the moment.

flake/default.nix

@@ -13,6 +13,7 @@ flake-parts.lib.mkFlake { inherit inputs; } {
     ./checks.nix
     ./dev-shells.nix
     ./home-manager.nix
+    ./hosts.nix
     ./lib.nix
     ./nixos.nix
     ./overlays.nix

flake/hosts.nix

@@ -0,0 +1,21 @@
+# Define `hosts.{darwin,home,nixos}` options for consumption in other modules
+{ lib, ... }:
+let
+  mkHostsOption = description: lib.mkOption {
+    inherit description;
+    type = with lib.types; attrsOf str;
+    default = { };
+    example = { name = "x86_64-linux"; };
+  };
+in
+{
+  options = {
+    hosts = {
+      darwin = mkHostsOption "Darwin hosts";
+
+      homes = mkHostsOption "Home Manager hosts";
+
+      nixos = mkHostsOption "NixOS hosts";
+    };
+  };
+}