home-manager: add module

This module is a module to install and configure the home-manager
tool. By managing the home-manager tool through the Home Manager
module system it will be installed/updated on configuration
activation.

(cherry picked from commit 7a18a0fb34)

modules/default.nix

@@ -19,6 +19,7 @@ let
     ./programs/firefox.nix
     ./programs/git.nix
     ./programs/gnome-terminal.nix
+    ./programs/home-manager.nix
     ./programs/info.nix
     ./programs/lesspipe.nix
     ./programs/ssh.nix

modules/programs/home-manager.nix

@@ -0,0 +1,39 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.programs.home-manager;
+
+in
+
+{
+  options = {
+    programs.home-manager = {
+      enable = mkEnableOption "Home Manager";
+
+      modulesPath = mkOption {
+        type = types.nullOr types.str;
+        default = null;
+        example = "$HOME/devel/home-manager/modules";
+        description = ''
+          The default path to use for Home Manager modules. If this
+          path does not exist then
+          <filename>$HOME/.config/nixpkgs/home-manager/modules</filename>
+          and <filename>$HOME/.nixpkgs/home-manager/modules</filename>
+          will be attempted.
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    home.packages = [
+      (import ../../home-manager {
+        inherit pkgs;
+        inherit (cfg) modulesPath;
+      })
+    ];
+  };
+}