Localize namespace-filtering code

Localize namespace-filtering code

To be able to filter results coming from
elisp-xref-find-def-functions, and for general ease of
understanding.

* lisp/progmodes/elisp-mode.el (elisp--xref-find-definitions):
Undo the previous change.
(xref-backend-apropos): Update accordingly.
(elisp--xref-filter-definitions): New function.
(xref-backend-definitions): Use it to post-filter the results
coming from elisp--xref-find-definitions.

* test/lisp/progmodes/elisp-mode-tests.el (find-defs-minor-defvar-c):
New test.
(find-defs-defun-defvar-el): Update test.