* lisp/ox-linguistics.el: simplify org-linguistics-linguex-item
  (org-linguistics-linguex-item): use new(?) functions
  org-export-first-sibling-p and org-export-last-sibling-p, provided
  by Org's exporter, in place of inline functions
* lisp/ox-linguistics.el: remove dependency on cl
  (org-linguistics-gb4e-item): replace unnecessary (labels ...)
  with (flet ...)
* lisp/ox-linguistics.el: complete ATTR_LINGUISTICS conversion
  (org-linguistics-item): check :attr_linguistics, not :attr_latex for
    :package attribute
  (forgot to stage this in 0c90fe04)