Nice work on this - looks very helpful!

One small suggestion, in some cases when creating a lambda layer, we may not want pull in dependencies when we pip install because we could be pulling them from another layer.

Example case in this blog post re numpy: https://medium.com/@qtangs/creating-new-aws-lambda-layer-for-python-pandas-library-348b126e9f3e

So I think an additional flag that is passed down to _make.sh to then invoke --no-deps on the pip install would be beneficial.