bug#47424: 28.0.50; enable-theme shows an empty list

[Stefan Monnier]

>> According to the documentation, `enable-theme' enables variable and face
>> settings of already loaded themes, so I think it makes sense that it
>> doesn't offer completions in emacs -Q, because there is no loaded theme
>> yet.
>>
>> Perhaps you wanted to do M-x load-theme, which should properly complete
>> with themes from etc/themes, among other directories?
>
> I know it works as documented, but I don't think it's the behavior we
> really want.  We should autoload themes, just like we autoload pretty
> much everything else.

BTW, we already do some of that for themes in other cases.
E.g. `M-x customize-themes` is perfectly able to list the available
themes without having to load them beforehand, and so is
`M-x custom-theme-visit-theme` which will additionally autoload the
theme you choose to customize if needed.


        Stefan