Add make_py_package_standalone.py PEX builder

[cormacrelf]

There is a reference in the prelude to make_py_package_standalone being a PEX archive builder for bundling a python binary together. The one Meta uses has never been open sourced. This is a version for OSS users. Fixes #229

This is a claude oneshot except for the zip_safe = True (default) behaviour.

Features:

• Pure execute from zip file behaviour for pure python
• If you have .so files in there it requires zip_safe = False passed to python_binary so you are aware it will use the self-extracting behaviour
• Self-extracting version extracts to ~/.cache/par/hello.pex-HASH with locking for multiple copies of the binary launched at the same time

To use:

buck2 build //my/python:binary[standalone]

# more complete example
cp $(buck2 build //my/python:binary[standalone] --show-full-simple-output) /tmp/binary.pex
/tmp/binary.pex

You can also use package_style = "standalone" on the target but I normally wouldn't as packaging a PEX is a quite intensive zip operation if you have a lot of dependencies with 100MB .so files etc, so you want the option to run using the link tree in the buck-out dir and not pay for the zip construction during development.

Basically this works pretty well. I think the zip stuff could be more deterministic than it is, but someone who needs that can make it so.

[meta-codesync]

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