* terraform: large refactor to use Provider from configs.Resource
configs.Resource.ImpliedProvider() now returns a string; it is the
callers' responsibility to turn that into an addrs.Provider if needed.
GraphNodeProviderConsumer ProvidedBy() no longer returns nil (reverting
to earlier, pre-provider-fqn behavior): it will return either the
provider set in config, provider set in state, or the default provider.
* configs: parse provider source string during module merge
This was the smallest unit of work needed to start writing provider
source tests!
* Update configs/parser_test.go
Co-Authored-By: Alisdair McDiarmid <alisdair@users.noreply.github.com>