Change xref-backend to xref-backend-function.

Using a function makes it easier to lazily load xref.el.

* xref.el (xref-backend-function): New
(xref-backend): Deleted.
(xref--backend): New function used instead of xref-backend.
(xref--elisp-backend, xref-elisp-backend-function): New.
(xref--etags-backend, xref-etags-backend-function): New.