Most legacy provider resources do not implement any import functionality
other than returning an empty object with the given ID, relying on core
to later read that resource and obtain the complete state. Because of
this, we need to check the response from ReadResource for a null value,
and use that as an indication the import id was invalid.
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.
