This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
This is an alternative to the full config loader in the "configs" package
that is good for "early" use-cases like "terraform init", where we want
to find out what our dependencies are without getting tripped up on any
other errors that might be present.