The owner of the repository has changed from segmentio to terraform-docs
since the last pull request.
This commit also introduces the use of buildGoModule in place of
buildGoPackage, so that we can avoid generating a deps.nix file for
module dependencies.
0.6.0 is quite an old version, that doesn't support Terraform 0.12. As
Terraform 0.12 is the default on master, upgrading terraform-docs to
0.9.1 brings tools on par.
