fix: improve nested stack check by using physical ids

[KyopoCanuck]

When resources have long names the ability to check for known tokens in the resulting generated names becomes unreliable. This updates the checks for nested stacks to:

• Collect nested stack physical ID's by searching through every stacks resource map for CloudFormation stack resources
• Filter the list of all stacks to down to nested stacks by checking the stack name against the list of nested stack physical ID's, i.e. physical ID contains string :stack/STACK_NAME/

--

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[rycerrat]

LGTM!