emacs: config: Use "emacs-config-" prefix for constants.

* emacs/guix-config.el.in (guix-emacs-interface-directory,
  guix-state-directory): Rename to ...
  (guix-config-emacs-interface-directory, guix-config-state-directory):
  ...this.
  (guix-config-guile-program): New constant.
  (guix-guile-program): Move to ...
* emacs/guix-external.el (guix-guile-program): ...here.  Make it a
  'defcustom'.
* emacs/guix-profiles.el (guix-default-profile): Use
  'guix-config-state-directory'.
* emacs/guix-backend.el (guix-load-path): Use
  'guix-config-emacs-interface-directory'.

emacs/guix-backend.el

@@ -56,9 +56,10 @@
 (require 'geiser-guile)
 (require 'guix-geiser)
 (require 'guix-config)
+(require 'guix-external)
 (require 'guix-emacs)
 
-(defvar guix-load-path guix-emacs-interface-directory
+(defvar guix-load-path guix-config-emacs-interface-directory
   "Directory with scheme files for \"guix.el\" package.")
 
 (defvar guix-helper-file

emacs/guix-config.el.in

@@ -20,21 +20,15 @@
 
 ;;; Code:
 
-(defconst guix-emacs-interface-directory
+(defconst guix-config-emacs-interface-directory
   (replace-regexp-in-string "${prefix}" "@prefix@" "@emacsuidir@"))
 
-(defconst guix-state-directory
+(defconst guix-config-state-directory
   ;; This must match `NIX_STATE_DIR' as defined in `daemon.am'.
   (or (getenv "NIX_STATE_DIR") "@guix_localstatedir@/guix"))
 
-(defvar guix-guile-program "@GUILE@"
-  "Name of the guile executable used for Guix REPL.
-May be either a string (the name of the executable) or a list of
-strings of the form:
-
-  (NAME . ARGS)
-
-Where ARGS is a list of arguments to the guile program.")
+(defconst guix-config-guile-program "@GUILE@"
+  "Name of the 'guile' executable defined at configure time.")
 
 (defconst guix-config-dot-program "@DOT_USER_PROGRAM@"
   "Name of the 'dot' executable defined at configure time.")

emacs/guix-external.el

@@ -29,6 +29,17 @@
   "Settings for external programs."
   :group 'guix)
 
+(defcustom guix-guile-program guix-config-guile-program
+  "Name of the 'guile' executable used for Guix REPL.
+May be either a string (the name of the executable) or a list of
+strings of the form:
+
+  (NAME . ARGS)
+
+Where ARGS is a list of arguments to the guile program."
+  :type 'string
+  :group 'guix-external)
+
 (defcustom guix-dot-program
   (if (file-name-absolute-p guix-config-dot-program)
       guix-config-dot-program

emacs/guix-profiles.el

@@ -26,7 +26,7 @@
   "User profile.")
 
 (defvar guix-default-profile
-  (concat guix-state-directory
+  (concat guix-config-state-directory
           "/profiles/per-user/"
           (getenv "USER")
           "/guix-profile")