Multi-file parsing examples

I've been considering playing with HCL2 for a project that would need to parse a directories full of inter-related HCL files, much like Terraform does. These files may have objects that are referring to one another across file boundaries. I see reference to how Terraform does this (at a high level) in the HCL1 docs, but I'm not quite fully understanding.

It would be wonderful to have some examples in the guide (ie: https://github.com/hashicorp/hcl2/blob/master/guide/go_parsing.rst) for how to handle this "parse a directory of HCL files" usage case. The single-file usage case seems super easy to tackle with the hcl2 parser, but I wonder if there might be others out there (like myself) who want the multi-file, TF-like behavior.

Start with guide/go_parsing.rst and the HCL1 documentation referenced in the issue, then review how Terraform handles directories of inter-related HCL files. Document a multi-file parsing example covering references across file boundaries, with enough guidance for users who understand the existing single-file example.】【。

Written by the indexing model from the issue text.