* lisp/cedet/ede.el: (project-try-ede): New function.
(project-root): New implementation.
* lisp/progmodes/elisp-mode.el (emacs-lisp-mode):
Set project-search-path-function.
(elisp--xref-find-references): Delegate some logic to
project-search-path.
(elisp-search-path): New function.
(elisp-xref-find): Don't implement `matches' anymore.
* lisp/progmodes/etags.el: Don't implement `matches'.
Delegate some logic to project-search-path.
(etags-search-path): New function.
* lisp/progmodes/xref.el (xref-find-function):
Remove `matches' from the API.
(xref-find-regexp): Move whatever common logic was in elisp and
etags implementations, and search the directories returned by
project-directories and project-search-path.