feature/lookup: fix +lookup/definition not fetching the correct identifier

Some xref backends have specialized thing-at-point implementations (such
as haskell's dante's xref backend), this fix uses them to fetch the
identifier at point.

modules/feature/lookup/autoload/lookup.el

@@ -59,7 +59,7 @@ to find it:
 
 Failing all that, it will give up with an error."
   (interactive
-   (list (thing-at-point 'symbol t)
+   (list (xref-backend-identifier-at-point (xref-find-backend))
          current-prefix-arg))
   (cond ((null identifier)
          (user-error "Nothing under point"))