emacs: list: Factorize "edit package" commands.

* emacs/guix-list.el: Use separate edit commands instead of a single one.
  (guix-list-for-each-line, guix-list-edit-package): Remove.
  (guix-package-list-edit, guix-output-list-edit): New procedures.
  (guix-package-list-mode-map, guix-output-list-mode-map): Use them.