The providers schema command is using the Config.ProviderTypes method,
which had not been kept up to date with the changes to provider
requirements detection made in Config.ProviderRequirements. This
resulted in any currently-unused providers being omitted from the
output.
This commit changes the ProviderTypes method to use the same underlying
logic as ProviderRequirements, which ensures that `required_providers`
blocks are taken into account.
Includes an integration test case to verify that this fixes the provider
schemas command bug.
18f9ea53b9