gnu: emacs-nix-mode: Move to package-management.scm.

Reported by Christopher Baines.
This fixes a top-level reference among mutually-dependent modules.

* gnu/packages/emacs.scm (emacs-nix-mode): Move to...
* gnu/packages/package-management.scm (emacs-nix-mode): ... here.

gnu/packages/emacs.scm

@@ -5258,23 +5258,6 @@ multiplexer.")
 editing RPM spec files.")
     (license license:gpl2+)))
 
-(define-public emacs-nix-mode
-  (package
-    (inherit nix)
-    (name "emacs-nix-mode")
-    (build-system emacs-build-system)
-    (arguments
-     `(#:phases
-       (modify-phases %standard-phases
-         (add-after 'unpack 'chdir-elisp
-           ;; Elisp directory is not in root of the source.
-           (lambda _
-             (chdir "misc/emacs"))))))
-    (synopsis "Emacs major mode for editing Nix expressions")
-    (description "@code{nixos-mode} provides an Emacs major mode for editing
-Nix expressions.  It supports syntax highlighting, indenting and refilling of
-comments.")))
-
 (define-public emacs-git-messenger
   (package
     (name "emacs-git-messenger")

gnu/packages/package-management.scm

@@ -2,6 +2,7 @@
 ;;; Copyright © 2013, 2014, 2015, 2016, 2017 Ludovic Courtès <ludo@gnu.org>
 ;;; Copyright © 2015, 2017 Ricardo Wurmus <rekado@elephly.net>
 ;;; Copyright © 2017 Muriithi Frederick Muriuki <fredmanglis@gmail.com>
+;;; Copyright © 2017 Oleg Pykhalov <go.wigust@gmail.com>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -26,6 +27,7 @@
   #:use-module (guix utils)
   #:use-module (guix build-system gnu)
   #:use-module (guix build-system python)
+  #:use-module (guix build-system emacs)
   #:use-module ((guix licenses) #:select (gpl2+ gpl3+ lgpl2.1+ asl2.0 bsd-3))
   #:use-module (gnu packages)
   #:use-module (gnu packages guile)
@@ -379,6 +381,23 @@ store, usually the directory /nix/store, where each package has its own unique
 sub-directory.")
     (license lgpl2.1+)))
 
+(define-public emacs-nix-mode
+  (package
+    (inherit nix)
+    (name "emacs-nix-mode")
+    (build-system emacs-build-system)
+    (arguments
+     `(#:phases
+       (modify-phases %standard-phases
+         (add-after 'unpack 'chdir-elisp
+           ;; Elisp directory is not in root of the source.
+           (lambda _
+             (chdir "misc/emacs"))))))
+    (synopsis "Emacs major mode for editing Nix expressions")
+    (description "@code{nixos-mode} provides an Emacs major mode for editing
+Nix expressions.  It supports syntax highlighting, indenting and refilling of
+comments.")))
+
 (define-public stow
   (package
     (name "stow")