magit: don't install lisp files into a "magit" sub-directory

After this change, Emacs can load the magit-*.el files without further
configuration. Just add

    (require 'magit-autoloads)

to ~/.emacs, and run M-x magit-status or any other magit command.

pkgs/applications/editors/emacs-modes/magit/default.nix

@@ -20,6 +20,7 @@ stdenv.mkDerivation rec {
     makeFlagsArray=(
       PREFIX="$out"
       EFLAGS="-L ${gitModes}/share/emacs/site-lisp"
+      lispdir="$out/share/emacs/site-lisp"
     )
   '';