Renamed file.ProviderRequirements to file.RequiredProviders to match the
name of the block in the configuration. file.RequiredProviders contains
the contents of the file(s); module.ProviderRequirements contains the
parsed and merged provider requirements.
Extended decodeRequiredProvidersBlock to parse the new provider source
syntax (version only, it will ignore any other attributes).
Added some tests; swapped deep.Equal with cmp.Equal in the
terraform/module_dependencies_test.go because deep was not catching
incorrect constraints.
