command/console: support root module output

[mildwonkey]

The terraform console command was unable to print root module outputs, instead only supporting module output (which I suspect won't work in 0.12 at all, since only root module outputs are stored in state).

Addresses some of the concerns in 21516, but possibly not all - as implemented this still requires an initial apply, even if the value is hard-coded in the module.

From a MODIFIED example in this comment:

echo output.stuff | terraform console
{
  "house" = {
    "foo" = "bar"
  }
}

I had to add the output to the root module for the above to work:

output "stuff" {
  value = module.stuff
}

Note that (as currently implemented) this only works after an apply; the value must be in state. If it's intended/expected to work without apply there's more to be done, but I hope this is a sufficient first step.

This works too!

echo output.stuff.house | terraform console
{
  "foo" = "bar"
}

[pselle]

From digging into outputs last week, I feel like this might not be the right direction (namely, making OutputValue referenceable, when it is noted to explicitly not be) to address this issue.

[pselle]

// OutputValue is not Referenceable, while ModuleCallOutput is.

The comment right above this would be incorrect from this PR, and as well, I wonder about how this relates to this code's intention.

[mildwonkey]

ah wonderful catch, thanks! The whole point is that it is now referenceable so MILDLY IMPORTANT that the comments match

[pselle]

Is this comment denoting that this is dead code?

[mildwonkey]

Yes, this function is required so that the interface is implemented, but is not used - I will add to the comment!

[mildwonkey]

@pselle thank you, this is really helpful feedback:

(namely, making OutputValue referenceable, when it is noted to explicitly not be)

I was only thinking about the console output needs and had missed the additional implications of making this referenceable. I'll look into alternate ways for the console to access root module outputs, which are not available in the ModuleCallOutput (and awkwardly, ModuleCallOutput is not available for console, so perhaps I can remove that from the console logic while I'm in there).

[ghost]

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