This new command prints out the tree of modules annotated with their
associated required providers.

The purpose of this command is to help users answer questions such as
"why is this provider required?", "why is Terraform using an older version
of this provider?", and "what combination of modules is creating an
impossible provider version situation?"

For configurations using many modules this sort of question is likely to
come up a lot once we support versioned providers.

As a bonus use-case, this command also shows explicitly when a provider
configuration is being inherited from a parent module, to help users to
understand where the configuration is coming from for each module when
some child modules provide their own provider configurations.