Re-organize and tidy up codegen packages

[haydenbaker]

Description of changes:

• Changes the codegen package structure, adds buildSrc plugins to clean up gradle configs
  • New packages will be easy to add with these changes
  • Removed aws package since it was effectively empty, can be re-added later
  • codegen-test package moved into the integration tests of core
• Adds spotless, formatted codegen
• Annotated all classes not labeled unstable as internal
• Adds instructions for setting up virtualenv with pants to ease development
• Updates Makefile to correctly display help info

Usage:
  make <target>

Targets:
  help                       Show this help.
  ...
  check-py                   Runs checkers for the python packages.
  ...
  clean                      Clean up generated code and artifacts.

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[JordonPhillips]

Needs to be rebased, but looks great! Thanks for doing this