refactor(systems): pull out inline modules

modules/system/host.nix

@@ -0,0 +1,105 @@
+{
+  name,
+  config,
+  meta,
+  std,
+  lib,
+  inputs,
+  ...
+}: let
+  inherit (lib.modules) mkOptionDefault;
+  inherit (std) string;
+in {
+  options = let
+    inherit (lib.types) str listOf attrs unspecified enum nullOr;
+    inherit (lib.options) mkOption;
+  in {
+    arch = mkOption {
+      description = "Processor architecture of the host";
+      type = str;
+      default = "x86_64";
+    };
+    type = mkOption {
+      description = "Operating system type of the host";
+      type = nullOr (enum ["NixOS" "MacOS" "Darwin" "Linux"]);
+      default = "NixOS";
+    };
+    folder = mkOption {
+      type = str;
+      internal = true;
+    };
+    system = mkOption {
+      type = str;
+      internal = true;
+    };
+    modules = mkOption {
+      type = listOf unspecified;
+    };
+    specialArgs = mkOption {
+      type = attrs;
+      internal = true;
+    };
+    builder = mkOption {
+      type = unspecified;
+      internal = true;
+    };
+    built = mkOption {
+      type = unspecified;
+      internal = true;
+    };
+  };
+  config = {
+    system = let
+      kernel =
+        {
+          nixos = "linux";
+          macos = "darwin";
+          darwin = "darwin";
+          linux = "linux";
+        }
+        .${string.toLower config.type};
+    in "${config.arch}-${kernel}";
+    folder =
+      {
+        nixos = "nixos";
+        macos = "darwin";
+        darwin = "darwin";
+        linux = "linux";
+      }
+      .${string.toLower config.type};
+    modules = [
+      # per-OS modules
+      meta.modules.${config.folder}
+      # per-OS configuration
+      meta.${config.folder}.base
+    ];
+    builder =
+      {
+        nixos = let
+          lib = inputs.nixpkgs.lib.extend (self: super:
+            import (inputs.arcexprs + "/lib") {
+              inherit super;
+              lib = self;
+              isOverlayLib = true;
+            });
+          sys = args:
+            lib.nixosSystem ({
+                inherit lib;
+              }
+              // args);
+        in
+          sys;
+        darwin = inputs.darwin.lib.darwinSystem;
+        macos = inputs.darwin.lib.darwinSystem;
+      }
+      .${string.toLower config.type};
+    built = mkOptionDefault (config.builder {
+      inherit (config) system modules specialArgs;
+    });
+    specialArgs = {
+      inherit name inputs std meta;
+      systemType = config.folder;
+      system = config;
+    };
+  };
+}