schema: Introduce CoreModuleSchemaForConstraint

[radeksimko]

This is to enable hashicorp/terraform-ls#993

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Implementation Notes

I chose to pull all known versions and generate a list of versions which we can check the constraints against. They are re-generatable at any time via go generate ./schema. This is more or less in-line with our longer term plans to have TF Core/CLI expose machine-readable schemas for core and publishing it somewhere for each version.

While working on this I also realised there's a few edge cases we didn't previously cover, which became more obvious now:

• what happens if the user has too old version/constraint we don't have schema for? (<0.12)
• what happens if the user has too new version/constraint we don't have schema for? (>= 1.3) - this is esp. important since we may not always have the list up-to-date and not all users may be on the latest version of LS while also consuming the latest version of Terraform

For both of these edge cases we return an error (which is what we did before), but given that we also now expose the lowest and highest (supported) version, we allow LS to pick either of those based on "proximity" to the version or constraint - e.g. if the user is running 0.5.0 we pick 0.12.0 as that's closest. If they run (hypothetical) 1.5.0, we pick 1.3.0.

[dbanck]

LGTM! 👍

Only the gen_test.go was a bit confusing to me