We've not been using HIL in the main codepaths since Terraform 0.12, but
some references to it (and some supporting functionality in Terraform)
stuck around due to interactions with types we'd kept around to support
legacy shims.

However, removing the configs.RawConfig field from
terraform.ResourceConfig disconnects that subtree of dependencies from
everything else, allowing us to remove it. This is safe because the only
remaining uses of terraform.ResourceConfig are shims from values that
were already evaluated using the HCL 2 API, and thus they never need
the "just in time" HIL evaluation that ResourceConfig.interpolateForce
used to do.

We also had some HIL references in configs/hcl2shim that were previously
in support of the "terraform 0.12upgrade" command, but the implementation
of that command is now removed.

There was one remaining reference to HIL in a now-unused function in the
helper/schema package, which I removed entirely here.

This then allows us to remove the HIL dependency entirely, and also to
clean up some remaining old remants of the legacy "config" package that
we'd recently moved into the "configs" package pending further pruning.