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'.