Add a setting to allow/disallow duplicate Terraform modules

Context

You can publish Terraform modules to your GitLab project. And you can install them from your group or sub-group.

Problem to solve

The problem is that a Developer in Project A may create a module called foo and this prevents Developers in other projects from using the name foo. And if they try they'll receive an error but have no way of knowing which project originally created foo.

Proposal

In some cases, organizations may want to allow duplicate package name/version combinations. GitLab offers a setting for this for Maven and generic packages. We will do the same for Terraform modules.