This was originally part of 7dad938fdb but
unfortunately seems to have got lost during some rebasing, or some other
similar sort of annoying reason.
This now allows the "experiments allowed" flag to propagate into the
stackeval package when we're running the apply phase, for consistency with
all of the other phases. Without this, it's possible to plan a
configuration that's participating in experiments, but then it fails in a
strange way during the apply step due to Terraform suddenly thinking it's
a stable release where experiments are disabled.
We allow experiments only in alpha builds, and so we propagate the flag
for whether that's allowed in from "package main". We previously had that
plumbed in only as far as the rpcapi startup.
This plumbs the flag all the way into package stackeval so that we can
in turn propagate it to Terraform's module config loader, which is
ultimately the one responsible for ensuring that language experiments can
be enabled only when the flag is set.
Therefore it will now be possible to opt in to language experiments in
modules that are used in stack components.
The closure of these channels is how a caller knows that the operation has
completed, so we must always close them before we return or the caller
is likely to get wedged.
