These changes refactor the engine's entrypoints--Deploy, Destroy, and
Preview--to be update-centric rather than stack-centric. Each of these
methods now takes a value of a new type, Update, that abstracts away the
vagaries of fetching and maintaining the update's state. This
refactoring also reinforces Pulumi.yaml as a CLI concept rather than an
engine concept; the CLI is now the only reader/writer of this format.
These changes will smooth the way for a few refactorings on the service
side that will aid in update isolation.
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