xref-find-definitions: Exclude more generic function items.

xref-find-definitions: Exclude more generic function items.

* lisp/emacs-lisp/cl-generic.el (cl--generic-search-method): Add doc string.
(cl--generic-find-defgeneric-regexp): New.
(find-function-regexp-alist): Add it.

* lisp/emacs-lisp/find-func.el (find-feature-regexp): Move here from
elisp-mode.el, change to search for ";;; Code:"
(find-alias-regexp): Move here from elisp-mode.el, cleaned up.
(find-function-regexp-alist): Add them.

* lisp/progmodes/elisp-mode.el:
(elisp--xref-format, elisp--xref-format-extra): Change back to defvar due
to bug#21237.
(elisp--xref-find-definitions): Exclude co-located default methods for
generic functions. Also exclude implicitly declared defgeneric.
(elisp--xref-find-definitions): Handle C source properly. Exclude minor
mode variables defined by 'define-minor-mode'.

* test/automated/elisp-mode-tests.el: Declare generic functions, add
tests for them.
(xref-elisp-test-run): Fix bug.
(emacs-test-dir): Improve initial value.
(find-defs-defun-defvar-el): Don't expect defvar.
(find-defs-feature-el): Match change to find-feature-regexp.