Add links to commentary reached with finder-list-keywords

Add links to commentary reached with finder-list-keywords

* lisp/finder.el (finder-goto-xref): Move from here...
* lisp/emacs-lisp/package.el (package--finder-goto-xref): ...to
here.  Make the old name into an obsolete function alias.
(package--finder-xref): New button type.
(package--describe-add-library-links): Factor out new function...
* lisp/finder.el (finder-commentary): ...from here.
(describe-package-1): Call above new function.  This fixes an
issue where commentaries reached via 'finder-list-keywords' did
not have links.  (Bug#10814)