emacs: Add 'list-single' variables.

* emacs/guix-list.el (guix-list-single-entry?): New procedure.
  (guix-list-define-interface): Add ':list-single?' key and generate
  'guix-ENTRY-TYPE-list-single' variables.
* emacs/guix-ui-package.el (guix-package-list-single): Remove.