emacs: Use package/output type variables where needed.

* emacs/guix-list.el (guix-package-list-type, guix-output-list-describe-type):
  Move to...
* emacs/guix-base.el: ... here.  Rename 'guix-output-list-describe-type' to
  'guix-package-info-type'.
* emacs/guix-info.el (guix-generation-info-insert-number): Use
  'guix-package-list-type'.
* emacs/guix.el (guix-get-show-packages): Use 'guix-package-info-type'.