home-manager: use `NIX_PATH` to locate modules

This removes the old argument based method that Home Manager used to
find its modules by a `NIX_PATH` based method. Specifically, this adds
a `home-manager` Nix path prefix that can be overridden much like with
the `nixpkgs` path prefix.

home-manager/default.nix

@@ -3,10 +3,10 @@
 let
 
   homeManagerExpr = pkgs.writeText "home-manager.nix" ''
-    { pkgs ? import <nixpkgs> {}, confPath, modulesPath }:
+    { pkgs ? import <nixpkgs> {}, confPath }:
 
     let
-      env = import modulesPath {
+      env = import <home-manager> {
         configuration = import confPath;
         pkgs = pkgs;
       };

home-manager/home-manager

@@ -41,6 +41,8 @@ function doBuild() {
         exit 1
     fi
 
+    export NIX_PATH="$NIX_PATH${NIX_PATH:+:}home-manager=@MODULES_PATH@"
+
     local extraArgs
     extraArgs=""
 
@@ -54,7 +56,6 @@ function doBuild() {
 
     nix-build $extraArgs \
               "@HOME_MANAGER_EXPR_PATH@" \
-              --argstr modulesPath "@MODULES_PATH@" \
               --argstr confPath "$confFile" \
               -A activation-script \
               -o "$output"