We want to report the result of component expansion to the client as
early as possible, so that it can prepare to receive any following
information for each instance. This message is critical when a
component's expansion expression evaluates to an empty set: without this
message's confirmation of this case, it will be unable to determine that
the component will receive no plans for any instances until the plan
completes.
We also want to report a roll-up of the resource changes upon completion
of each component's plan and apply operations, so that a client can
summarize the operation changes without performing the roll-up operation
on all received change events.
