home-manager: point <home-manager> to project root

Before this path would point to the modules path. Using the project
root instead makes it possible to set `<home-manager>` to point to a
downloadable archive of Home Manager. This should make it
significantly easier to install and keep Home Manager up to date.

To match this change we also deprecate the Home Manager option

    programs.home-manager.modulesPath

and instead ask users to use

    programs.home-manager.path

default.nix

@@ -1,2 +1,14 @@
-# Simply defer to the home-manager script derivation.
-import ./home-manager
+{ pkgs ? import <nixpkgs> {} }:
+
+rec {
+  home-manager = import ./home-manager {
+    inherit pkgs;
+    path = ./.;
+  };
+
+  install =
+    pkgs.runCommand
+      "home-manager-install"
+      { propagatedBuildInputs = [ home-manager ]; }
+      "";
+}