emacs: info: Split 'guix-info-format' variable.

* emacs/guix-info.el: Generate 'guix-ENTRY-TYPE-info-format' variables
  for 'package', 'installed-output', 'output' and 'generation' entry
  types.
  (guix-info-format): Remove.
  (guix-info-data): New variable.
  (guix-info-value): New procedure.
  (guix-info-define-interface): Add ':format' keyword.
* emacs/guix-base.el (guix-buffer-define-interface): Add ':reduced?'
  keyword.
* doc/emacs.texi (Emacs Appearance): Adjust accordingly.