bug#44529: [PATCH] Convert apropos-internal from C to Lisp

[Eli Zaretskii]

> From: Stefan Monnier <monnier@iro.umontreal.ca>
> Cc: Stefan Kangas <stefan@marxist.se>,  44529@debbugs.gnu.org
> Date: Tue, 10 Nov 2020 08:14:01 -0500
> 
> > Because if we don't, every Emacs session will load it right at the
> > start.
> 
> I don't see that: elisp-mode.el calls `apropos-internal` only from
> `xref-backend-apropos`, which AFAICT is not used "right at the
> start" but only after loading `xref.el` which is itself not preloaded.

I don't want to rely on such shaky assumptions: the code in
elisp-mode.el can change any moment.

apropos-internal was always available in Emacs, so let's leave it
always available by putting it in subr.el or somesuch.