YAML anchor unrolling

[shortjared]

Describe your idea/feature/enhancement

This issue is to track/confirm that AWS CLI will behave the same as AWS CLI in terms of packaging resolving CloudFormation YAML anchors/aliases. aws/aws-cli#3827

[sanathkr]

Oh thanks for the Issue. We have a copy of yamlhelper.py (surprise!) in SAM CLI that we now need to update. "sam package/deploy" commands wrap AWS CLI commands, so they should continue to work, but other parts of the CLI might not.

Would you be able to send a PR? It should just be some copy/pastas :)