home-manager: allow a user-defined third module path

The user-defined path will be used if present, otherwise the two
"fallback" defaults (in `.nixpkgs` and `.config/nixpkgs`) will be
used.

home-manager/default.nix

@@ -1,4 +1,10 @@
-{ pkgs, modulesPath ? "$HOME/.config/nixpkgs/home-manager/modules" }:
+{ pkgs
+
+  # Extra path to the Home Manager modules. If set then this path will
+  # be tried before `$HOME/.config/nixpkgs/home-manager/modules` and
+  # `$HOME/.nixpkgs/home-manager/modules`.
+, modulesPath ? null
+}:
 
 let
 
@@ -16,6 +22,8 @@ let
       }
   '';
 
+  modulesPathStr = if modulesPath == null then "" else modulesPath;
+
 in
 
 pkgs.stdenv.mkDerivation {
@@ -29,7 +37,7 @@ pkgs.stdenv.mkDerivation {
     substituteInPlace $out/bin/home-manager \
       --subst-var-by bash "${pkgs.bash}" \
       --subst-var-by coreutils "${pkgs.coreutils}" \
-      --subst-var-by MODULES_PATH '${modulesPath}' \
+      --subst-var-by MODULES_PATH '${modulesPathStr}' \
       --subst-var-by HOME_MANAGER_EXPR_PATH "${homeManagerExpr}"
   '';
 

home-manager/home-manager

@@ -39,6 +39,7 @@ function setConfigFile() {
 function setHomeManagerModulesPath() {
     local modulesPath
     for modulesPath in "@MODULES_PATH@" \
+                       "$HOME/.config/nixpkgs/home-manager/modules" \
                        "$HOME/.nixpkgs/home-manager/modules" ; do
         if [[ -e "$modulesPath" ]] ; then
             export NIX_PATH="$NIX_PATH${NIX_PATH:+:}home-manager=$modulesPath"