An unknown block represents a dynamic configuration block with an
unknown for_each value. We were not catching the case where a provider
modified this value unexpectedly, which would crash with block of type
NestingList blocks where the config value has no length for comparison.
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.