json-output: Add resource drift to machine readable UI

[alisdair]

The new resource_drift message allows consumers of the terraform plan -json output to determine which resources Terraform found to have changed externally after the refresh phase.

Note to reviewers: this implementation is a third copy of the logic for determining resource drift for a plan. I'm not happy about this, but I'm struggling to see a way to extract the logic to a single place, given the intertwingled dependencies. Any suggestions?

[apparentlymart]

Functionality-wise, this looks great to me.

I share your concern but also the uncertainty about how better to factor the logic for taking two states and detecting drift from them. Given the circular-import challenges we had with trying to reuse the types from plans, the best idea I have to hand right now is to establish a new package plans/drift which defines some types similar to some of the model types we have in plans to model changes.

I do wonder about potentially factoring out the change-model-related types into a separate plans/changes package which various other packages could then call on, but that'd be a pretty noisy change in terms of how many users it would update, and also the models in plans are tailored for the specific need of serializing changes into a plan file, and we don't really have those needs here because we can always re-derive the same answer from the pair of state snapshots we already have serialized in there.

Given that, maybe it's not too bad to have some similar-but-not-identical model types specialized for representing drift, particularly if we make an effort to keep them as similar as makes sense to be in case we find a path to converging them later.

🤔

[alisdair]

I like the plans/drift idea, and would like to pursue that. As I'm still targeting a 1.0 backport for this fix, I won't make that change in this PR, which I'd like to keep as non-invasive as possible.

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