Change xref-find-apropos to pass PATTERN to backend verbatim

Change xref-find-apropos to pass PATTERN to backend verbatim

* lisp/progmodes/xref.el (xref-backend-apropos): Rename this
generic's second arg to PATTERN, to clarify that it should be
handled entirely in the backend, with no pre-processing by the
command.
(xref-find-apropos): Update accordingly, but keep compatibility
with backends in older Emacs versions.
(xref-apropos-regexp): Extract from xref-find-apropos.

* lisp/progmodes/etags.el (xref-backend-apropos): Use it here.

* lisp/progmodes/elisp-mode.el (xref-backend-apropos): And here.