Use the new HCL2 config loader when the opt-in comment #terraform:hcl2 is
present in a .tf file.
For now this is disabled for "normal" builds and enabled only if
explicitly configured via a linker flag during build. This is because it's
not yet in a good state to be released: the HCL2 loader produces RawConfig
objects that the validator and interpolator can't yet deal with, and so
using HCL2 for anything non-trivial currently causes Terraform to crash
in real use.
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