Restore xref-based jump-to-definition in Babashka

[vemv]

Fixes #3456

I tested this one locally by cider-connecting to a lein repl session without cider-nrepl in it.

M-x xref-find-definitions now works (and does not work without this patch applied).

Cheers - V

[vemv]

We weren't actually using ns-path under this code path.

We only hit cider-var-info, which looks like this:

(defun cider-var-info (var &optional all)
  "Return VAR's info as an alist with list cdrs.
When multiple matching vars are returned you'll be prompted to select one,
unless ALL is truthy."
  (when (and var (not (string= var "")))
    (let ((var-info (cond
                     ((cider-nrepl-op-supported-p "info") (cider-sync-request:info var))
                     ((cider-nrepl-op-supported-p "lookup") (cider-sync-request:lookup var))
                     (t (cider-fallback-eval:info var)))))
      (if all var-info (cider--var-choice var-info)))))

So the lookup fallback already was there.