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.
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