Packages lambdas with intra-project dependencies using Webpack and a base layer with the dependencies defined in package.json.
Checksums are created for dependencies per lambda so that rebuild only happens when files are changed.
Packaged lambdas are published to S3 so they can be picked up and deployed by CloudFormation. It's also a convenient cache for other developers.
More background information on this project and usage instructions can be found in this blog post.