This switches to the Go "context" package for cancellation and threads
the context through all the way to evaluation to allow behavior based on
stopping deep within graph execution.
This also adds the Stop API to provisioners so they can quickly exit
when stop is called.
People with `uuid()` usage in their configurations would receive shadow
errors every time on plan because the UUID would change.
This is hacky fix but I also believe correct: if a shadow error contains
uuid() then we ignore the shadow error completely. This feels wrong but
I'll explain why it is likely right:
The "right" feeling solution is to create deterministic random output
across graph runs. This would require using math/rand and seeding it
with the same value each run. However, this alone probably won't work
due to Terraform's parallelism and potential to call uuid() in different
orders. In addition to this, you can't seed crypto/rand and its unlikely
that we'll NEVER use crypto/rand in the future even if we switched
uuid() to use math/rand.
Therefore, the solution is simple: if there is no shadow error, no
problem. If there is a shadow error and it contains uuid(), then ignore
it.
This is necessary so that the shadow version can actually keep track of
what provider is used for what. Before, providers for different alises
were just initialized but the factory had no idea. Arguably this is fine
but when trying to build a shadow graph this presents challenges.
With these changes, we now pass an opaque "uid" through that is used to
keep track of the providers and what real maps to what shadow.
