Providers can be required from multiple sources. The previous
implementation of the providers sub-command displayed only a flat list
of provider requirements, which made it difficult to see which modules
required each provider.
This commit reintroduces the tree display of provider requirements, and
adds a separate output block for providers required by existing state.
The providers command has been refactored to use the modern provider types and
ProviderRequirements() functions. This resulted in a breaking change to
the output: it no longer outputs the providers by module and no longer
prints `(inherited)` or `(from state)` to show why a provider is
included. We decided that at this time it was best to stick with the
existing functions and make this change, but if we get feedback from the
community we will revisit.
Additional tests to exercise providers in modules and providers from
state have been included.
Alisdair McDiarmid