refactor(nixos): change hostSpec to define top level option

modules/configurations/nixos.nix

@@ -0,0 +1,28 @@
+{ config, lib, ... }:
+let
+  inherit (lib.trivial) pipe;
+  inherit (lib.attrsets) filterAttrs mapAttrs';
+  inherit (lib.strings) removePrefix hasPrefix;
+  prefix = "nixos/";
+  hosts = config.flake.hostSpec.hosts or { };
+  mkSystem =
+    name: value:
+    let
+      hostName = removePrefix prefix name;
+    in
+    {
+      name = hostName;
+      value = lib.nixosSystem {
+        modules = [
+          value
+          { networking = { inherit hostName; }; }
+        ];
+      };
+    };
+in
+{
+  flake.nixosConfigurations = pipe hosts [
+    (filterAttrs (name: _: hasPrefix prefix name))
+    (mapAttrs' mkSystem)
+  ];
+}

modules/hostSpec.nix

@@ -0,0 +1,3 @@
+{
+  flake.hostSpec.hosts = { };
+}

modules/hostSpec/default.nix

@@ -1,29 +0,0 @@
-{
-  lib,
-  config,
-  inputs,
-  ...
-}:
-let
-  inherit (builtins) mapAttrs;
-  inherit (lib) mkOption;
-  inherit (lib.types) lazyAttrsOf submodule;
-  cfg = config.hostSpec;
-  hostOptions = submodule { };
-  mkSystem =
-    hostName: _hostConfig:
-    inputs.nixpkgs.lib.nixosSystem {
-      system = "x86_64-linux";
-      modules = [ { networking = { inherit hostName; }; } ];
-    };
-in
-{
-  options.hostSpec.hosts = mkOption {
-    type = lazyAttrsOf hostOptions;
-    default = { };
-  };
-
-  config = {
-    flake.nixosConfigurations = mapAttrs mkSystem cfg.hosts;
-  };
-}