Add test coverage of dataproc PKG bundling.

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We have custom logic that manages the pip bundling for dataproc. Write a test that validates it.

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Also a reminder to keep an eye on hail-is/hail#14843 and onnx/onnx#4475 getting merged into a usable hail and onyx-converter versions. They have conflicting pinned versions requiring manual workarounds in our management.