bug#57955: 29.0.50; Allow session-local ERC modules

[J.P.]

"J.P." <jp@neverwas.me> writes:

> I'd like to propose that these changes be included in ERC 5.5 and Emacs
> 29. If anyone has any concerns, please speak up. Otherwise, expect this
> patch to be installed at some point as part of the larger SASL change
> set in bug#29108. Thanks.

Brief update. The scope of this change has shrunk considerably. The
deferred loading and migrations stuff still applies, but the main
user-facing aspect, namely, support for let-binding a module's options
on entry-point invocation, has been abandoned (for now).

After looking into connection-local variables a bit, following Michael's
suggestion up thread, I have come to the opinion that the let-binding
idea was not fully formed and that options granularity is worthy of more
meditation and discussion and thus not a realistic goal for Emacs 5.5.

Thanks.