A user could have a monorepo full of sibling modules. Or, they could have a root module and then modules/child called in the root module. In this case, modules will return both the root and the child, but root-modules would only return the root. Some tools will want to run on all modules (e.g., TFLint) while others should only run on roots (terraform validate).

Determining root modules requires parsing the configuration to find the source value for module calls. That will probably require https://github.com/hashicorp/terraform-config-inspect and may justify converting this action to Go.

Matrix inputs cannot accept an empty value (''). When a found dir is an empty string, . or ./ should be returned. Perhaps all relative paths should begin with ./ for consistency.