Re-implement metadata yaml file in ZIP #6
Description
The metadata YAML file emitted to the ZIP was removed to support de-duplication for layers.
This can be re-added. Additionally, the directory where lambda_zip is being used should be added to metadata. The various methods & functions for gathering & organizing metadata should be refactored since we now have two groups of metadata fields: those which fit into the Description, and those that are too large, and are only going into the YAML file. This should be more clear and easy to understand & maintain.