This changes a few aspects of resource properties:
* Move all runtime-related goo into the runtime module, in an
internal PromiseState class. This encapsulates the internal
state transitions and protects against misuse. It also allows
us to clean up the public API for the Property<T> type so that
it's entirely suitable for external usage.
* Track input and output property values distinctly. It turns
out we want to key off events differently. For example, to marshal
property values to a resource provider, we only care about the
inputs. For final property values that are used in, say, thens
or as inputs to other properties, we want the output property value.
* Be more precise about when an output is truly final, and known, or
unknown due to planning/dry-runs. Note that this does mean that
we'll encounter unknown values more frequently because, aside from
IDs and URNs, we can't say for sure that arbitrary properties will never
change post-creation. We have ideas on how to denote this; see
pulumi/pulumi-fabric#330 for more details.
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