Due to how deeply the configuration types go into Terraform Core, there
isn't a great way to switch out to HCL2 gradually. As a consequence, this
huge commit gets us from the old state to a _compilable_ new state, but
does not yet attempt to fix any tests and has a number of known missing
parts and bugs. We will continue to iterate on this in forthcoming
commits, heading back towards passing tests and making Terraform
fully-functional again.

The three main goals here are:
- Use the configuration models from the "configs" package instead of the
  older models in the "config" package, which is now deprecated and
  preserved only to help us write our migration tool.
- Do expression inspection and evaluation using the functionality of the
  new "lang" package, instead of the Interpolator type and related
  functionality in the main "terraform" package.
- Represent addresses of various objects using types in the addrs package,
  rather than hand-...