AUCTeX and "builtin latex mode" integration

[Stefan Monnier]

You may have seen an on-going discussion about the annoyance of the
mixups between latex-mode and LaTeX-mode and such.

I do think it would be good to try and "integrate" the two modes, for
some definition of "integrate".  The question being indeed what
"integration" is the best way forward.

I'm wondering if it would be possible to turn AUCTeX into a minor mode.
E.g. `LaTeX-mode` would then be redefined as

    (define-derived-mode TeX-latex-mode latex-mode ...
      (AUCTeX-mode 1))

and `TeX-modes-set` would add/remove `AUCTeX-mode` from the hooks like
`latex-mode-hook`.

To be honest, I haven't looked very much at the code to see how
realistic this is, but I think this could help clarify the situation for
users.  E.g. it would eliminate "lies" such as (setq major-mode
'latex-mode) in `latex.el`.  It would also mean that `TeX-latex-mode`
runs `latex-mode-hook`.  There's no doubt that it would introduce some
(presumably minor or easy to fix) breakage in some corner cases for some
exotic configs, of course, but I feel like in the long run it might be
worth it.

WDYT?


        Stefan