backend/local: Require caller to set PlanOutBackend with PlanOutPath
backend/local: Require caller to set PlanOutBackend with PlanOutPath

We can't generate a valid plan file without a backend configuration to
write into it, but it's the responsibility of the caller (the command
package) to manage the backend configuration mechanism, so we require it
to tell us what to write here.

This feels a little strange because the backend in principle knows its
own config, but in practice the backend only knows the _processed_ version
of the config, not the raw configuration value that was used to configure
it.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk