Now that the terraform 0.12 compatibility is not needed anymore, the
`terraform.withPlugins` and `terraform-providers.mkProvider`
implementations can be simplified.
Instead of building a bunch of bin wrappers on instantiation, the
providers are now stored in
`$out/libexec/terraform-providers/<registry>/<owner>/<name>/<version>/<os>_<arch>/terraform-provider-<name>_v<version>`
and then a simple `buildEnv` can be used to merge them.
This breaks back-compat so it's not possible to mix-and-match with
previous versions of nixpkgs. In exchange, it now becomes possible to
use the providers from
[nixpkgs-terraform-providers-bin](https://github.com/numtide/nixpkgs-terraform-providers-bin)
directly.