On IRC (#emacs-beginners) they suggested me to add this function to
~/.emacs:

(define-key tabulated-list-mode-map (kbd "s") 'tabulated-list-sort)
(defun tabulated-list-sort (column-name)
  (interactive
   (list (completing-read "Column: "
			  (mapcar 'car tabulated-list-format))))
  (tabulated-list--sort-by-column-name column-name))

I haven't learned Emacs Lisp yet so I can't help but this may be a
starting point for implementing this feature.

The previous function seems to work fine for all columns except for
Description and Version, but I don't know if it makes sense to sort
packages according to those columns.

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