Terraform 0.13 will allow the installation of providers from various
sources. If a user updates their configuration to change the source of
an in-use provider (for example, if the provider namespace changes),
they will also need to update the state file accordingly.
This commit introduces a new `state replace-provider` subcommand which
supports this. All resources using the `from` provider will be updated
to use the `to` provider.
Alisdair McDiarmid