[bug#33598] Optimizations for emacs-clang-format and emacs-clang-rename

[Pierre Neidhardt]

The benefit of separate Emacs packages is if the Emacs library can be used
without installing the parent package to the user profile.

For instance, emacs-clang-rename can be installed and it will work while the
user does not have to install clang.  (Clang remains an input, obviously.)

For this reason, "package-elisp-from-package" gives maximal flexibility in my
opinion.

Currently, there are a few more packages.  We can look up "emacs-build-system"
outside emacs.scm to find them (e.g. agda2).

Off the top of my head, there is also asymptote.

Now to the ideal place for package-elisp-from-package: it seems that no existing
file would be a fit.  So what about guix/utils/emacs.scm?  Having a separate
file would make sure we run into other meta-circular dependency issues.