* configs: added map of ProviderLocalNames to configs.Module
We will need to lookup any user-supplied local names for a given FQN.
This PR adds a map of ProviderLocalNames to the Module, along with
adding tests for this and for decodeRequiredProvidersBlock.
This also introduces the appearance of support for a required_provider
"source" attribute, but ignores any user-supplied source and instead
continues to assume that addrs.NewLegacyProvider is the way to go.
Kristin Laemmert