This change brings back component outputs to the overall system again.
In doing so, it generally overhauls the way we do resource RPCs a bit:
* Instead of RegisterResource and CompleteResource, we call these
BeginRegisterResource and EndRegisterResource, which begins to model
these as effectively "asynchronous" resource requests. This should also
help with parallelism (https://github.com/pulumi/pulumi/issues/106).
* Flip the CLI/engine a little on its head. Rather than it driving the
planning and deployment process, we move more to a model where it
simply observes it. This is done by implementing an event handler
interface with three events: OnResourceStepPre, OnResourceStepPost,
and OnResourceComplete. The first two are invoked immediately before
and after any step operation, and the latter is invoked whenever a
EndRegisterResource comes in. The reason for the asymmetry here is
that the checkpointing logic in the deployment engine is largely
untouched (intentionally, as this is a sensitive part of the system),
and so the "begin"/"end" nature doesn't flow through faithfully.
* Also make the engine more event-oriented in its terminology and the
way it handles the incoming BeginRegisterResource and
EndRegisterResource events from the language host. This is the first
step down a long road of incrementally refactoring the engine to work
this way, a necessary prerequisite for parallelism.
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