Rewrite elisp--xref-find-definitions to handle many more cases; add tests.
* lisp/progmodes/elisp-mode.el (elisp--xref-identifier-location): deleted
(elisp--xref-format-cl-defmethod): new
(find-feature-regexp): new
(find-alias-regexp): new
(elisp--xref-make-xref): new
(elisp--xref-find-definitions): Rewrite using the above, handle many more
cases. Always output all available definitions.
(xref-location-marker): No need for special cases.
* test/automated/elisp-mode-tests.el: Add more tests of
elisp--xref-find-definitions, improve current tests.