Support terraform resources outside the given template directory #5677

When creating a Coder template, I would like to reference local files outside the given template directory.

Context: I want to re-use common files/terraform modules between multiple templates in a repository. To do this right now, one needs to copy-paste common resources into each template's directory, which is inconvenient and increases the risk of introducing skew.

A practical example: These two templates are near-identical, yet it's not feasible to share common resources between then

• https://github.com/denbeigh2000/coder-templates/blob/master/aws-spot-nixos/main.tf
• https://github.com/denbeigh2000/coder-templates/blob/master/aws-nixos/main.tf