Managing which function need to be shared between the terraform plugin
and the helper plugin without creating cycles was becoming difficult.
Move all functions related to converting between terraform and proto
type into plugin/convert.
The new helper/plugin package contains the grpc servers for handling the
new plugin protocol
The GRPCProviderServer and GRPCProvisionerServer handle the grpc plugin
protocol, and convert the requests to the legacy schema.Provider and
schema.Provisioner methods.
Martin Atkins