[mu4e] Fallback to loading mu4e-autoloads if package not activated

[bcc32]

Fix #16931.

Rather than just reverting the previous change and loading mu4e at
startup, just load its autoloads. Warn if they were not already
loaded by package-activate, to encourage users to fix issues in
their Emacs site-lisp distribution or report issues to their
distribution maintainers.

[fnussbaum]

As in the factor case, some distributions (I noticed this on Ubuntu 22.04) do not even provide the autoloads file. Should we fall back to eager loading in that case? Or display a different warning, perhaps even mentioning how to generate missing autoloads: (loaddefs-generate mu4e-installation-path "mu4e-autoloads.el").

[bcc32]

If the distribution does not provide the autoloads file, it's unlikely the user can generate the missing autoloads with loaddefs-generate (usually the install directories will be read-only, right? It's actually been a long time since I regularly used a non-NixOS Linux distribution so I don't know off-hand).

[fnussbaum]

Ah indeed, one would need to do something like (find-file mu4e-installation-path) and SPC f E first, only then loaddefs-generate succeeds.

(Of course, this only works with root access, so perhaps we should fall back to (require 'mu4e) if mu4e-autoloads is not found.)