* lisp/progmodes/xref.el (xref-find-function): Describe the
`matches' action.
(xref-find-regexp): New command, using it.
(xref-collect-references): Rename to xref-collect-matches.
(xref--collect-reference): Rename to xref--collect-match.
(xref-collect-matches, xref--collect-match): Accept new argument,
KIND. Update accordingly.
(xref--regexp-to-extended): New function.
* lisp/progmodes/elisp-mode.el (elisp-xref-find): Support the
`matches' action.
(elisp--xref-find-matches): Accept new argument. Resolve a FIXME.
* lisp/progmodes/etags.el (etags-xref-find):
Support the `matches' action.
(etags--xref-find-matches): New function.