bug#43476: feature/native-comp; path for .eln files when running with --no-init-file

[Tom Gillespie <tgbugs@gmail.com>]

Hi Andrea,
   Not quite a bug, but an inconsistency in the behavior of
comp-eln-load-path compared to the behavior of load-path. Best!
Tom

While investigating the json-mode kill-buffer bug I encountered an
issue which seems like it might cause confusion at some point.
comp-eln-load-path is set at startup and materialized.

When Emacs is launched with -q, load-path does not contain the
materialized locations of paths inside user-emacs-directory until
after a call to package-initialize.

I think that there is a similar need for comp-eln-load-path, which is
that it needs to exclude the user eln cache path when emacs is
launched with -q so that it is possible to debug issues arising from
stale/bad eln files. There will be a similar issue with site-lisp and
-Q.

I don't know whether stale files could cause a problem given how you
hash to generate the names for the eln files, but I'm imagining cases
where someone upgraded gcc or libgccjit and something changed. I think
that not setting the user path when Emacs is run with -q is consistent
with Emacs behavior for load-path.

Right now a user has to explicitly pop the old eln cache directory and
then update the new directory if they reset user-emacs-directory as I
do in the longer reproduction for the json-mode bug (reproduced
below).

#+begin_src bash
emacs -q \
--eval "(setq user-emacs-directory \"/tmp/test/.emacs.d/\")" \
--eval "(when (boundp 'comp-eln-load-path) (pop comp-eln-load-path)
(add-to-list 'comp-eln-load-path (concat user-emacs-directory \"eln-cache\")))"
#+end_src

I don't have a good solution for this, but wanted to raise the issue
since I anticipate that there will be quite a bit of confusion if the
user eln cache continues to point to the path ~/.emacs.d/eln-cache
instead of either not being set or updating to be inside
user-emacs-directory the first time the value is needed.