This continues the previous commit and establishes the interpreter
context so that we can use the new host interface. In summary:
* Instead of using the NullSource for destructions -- which
doesn't hook up an interpreter and so any reads of configuration
variables will fail -- we will enlighten the EvalSource to know
how to orchestrate destruction interpretation. The primary
difference is that we don't actually run the code, but *we do*
perform all of the necessary configuration and variable init.
* Associate the active interpreter with the plugin context as
we are executing, so that the host object can actually read the
state from the heap as requested to do so by attached plugins.
* Rename anything "engine" related to use the term "host"; this
avoids introducing unnecesarily new terminology.
* Add a new pkg/resource/provider/ package where we can begin
consolidating helper functionality for resource providers.
Right now, this includes a wrapper interface atop the gRPC
machinery necessary to contact the host, in addition to a
Main function that hides some boilerplate entrypoint code.
* Add a rpcutil.IsBenignCloseErr routine to let us ignore
"benign" gRPC errors that are knowingly returned at shutdown.
This commit completes pulumi/lumi#117.
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