Unbreak jumping to an alias's definition

* lisp/emacs-lisp/find-func.el (find-function-library): Return a pair
(ORIG-FUNCTION . LIBRARY) instead of just its second element.
(find-function-noselect): Use it.

* lisp/progmodes/elisp-mode.el (elisp--xref-identifier-file): Rename to
`elisp--xref-identifier-location', incorporate logic from
`elisp--xref-find-definitions', use the changed
`find-function-library' return value.