feat: py_image_layers

[alexeagle]

Fixes #212

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Changes are visible to end-users: yes

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): kind of - needs bazel_lib 2.x
• Suggested release notes appear below: yes

A helper function to create image layers for a py_binary is now included.

Test plan

• New test cases added

[mattem]

Please provide a description of the change rather than just a link to an issue.