Previously we were using fmt.Sprintf and thus forcing the stringification
of the wrapped error.
Using errwrap allows us to unpack the original error at the top of the
stack, which is useful when the wrapped error is really a hcl.Diagnostics
containing potentially-multiple errors and possibly warnings.
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.
