unused outputs in a destroy should be pruned

[jbardin]

During a full destroy when outputs are removed, the
NodeDestroyableOutput was preventing it's sibling output from being
destroyed. Prune the output node if it only has its destroy node as a
dependent.

The destroy output test is simply run a second time with no state, which
would cause the output interpolation to fail if it remained in the
graph.

Fixes #17691
Fixes #17655

[apparentlymart]

"Keep iterating until things stop changing" is my favorite type of algorithm in programming language implementations. 😀

[sulabhk]

I'm on Terraform V0.11.8 and get a similar error
TF_WARN_OUTPUT_ERRORS is set to 1, and see an interpolation error when running destroy on an already destroyed environment.

Referred as per the following log https://github.com/hashicorp/terraform/blob/master/CHANGELOG.md#0111-november-30-2017

7 error(s) occurred:

* module.gen-db.var.subnet_ids: Resource 'aws_subnet.first_private_subnet' does not have attribute 'id' for variable 'aws_subnet.first_private_subnet.id'
* module.gen-db.var.vpc_id: Resource 'aws_vpc.main' does not have attribute 'id' for variable 'aws_vpc.main.id'
* module.gen-db.var.ingress_security_groups: Resource 'aws_security_group.tomcat' does not have attribute 'id' for variable 'aws_security_group.tomcat.id'
* module.gen-db.var.vpc_security_group_ids: Resource 'aws_security_group.rds-sg' does not have attribute 'id' for variable 'aws_security_group.rds-sg.id'
* module.gen-db.var.zone_id: Resource 'aws_route53_zone.internal' does not have attribute 'zone_id' for variable 'aws_route53_zone.internal.zone_id'
* module.gen-db.var.kms_key_id: Resource 'aws_kms_key.rds' does not have attribute 'arn' for variable 'aws_kms_key.rds.arn'
* module.gen-db.var.db_subnet_group_name: Resource 'aws_db_subnet_group.default' does not have attribute 'id' for variable 'aws_db_subnet_group.default.id'```

[ghost]

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