(TFECO-7637) Raise HCL diagnostics

[jpogran]

This change updates the stack metadata loading to merge the new diagnostics with the existing ones. This is necessary to ensure that we don't lose any diagnostics that were previously reported.

Needs hashicorp/terraform-schema#412

[dbanck]

I noticed a few things when I tried to use this feature. These don't require changes to this PR, but rather to terraform-schema. I am sorry that I missed them during the review of hashicorp/terraform-schema#412 and am only bringing them up now.

Maybe we can add more test cases to the decoder in terraform-schema to cover them.

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We need to check that req.Source and req.VersionConstraints are both not null when iterating over the requirements, to avoid crashing due to a null pointer dereference.

Both diagnostic messages contain nil because we are using an empty field in our string (e.g. pr.source instead of source.AsString()).

[github-actions]

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