chore(semantics): Remove determine_main

[arrdem]

Vendor rules_python's determine_main and remove the need for a separate determine_main target which complicates exec properties.

Replaces #641 with thanks to Keith.
Fixes #605
Fixes #621

Note that py_venv_* already mandates that a main= label be provided, so this is for legacy py_binary only. Should be removed in 2.0.

Changes are visible to end-users: no

• Searched for relevant documentation and updated as needed: yes
• Breaking change (forces users to change their own code or config): no
• Suggested release notes appear below: no

Test plan

• Covered by existing test cases

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examples/uv_pip_compile

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[alexeagle]

note that this is a subtle compatibility break with rules_python, at least last time I dug into this code, rules_python users couldn't adopt rules_py without changing their code.

Perhaps we can make the case that we no longer aim for drop-in compatibility with rules_python, but if we give that up we ought to be clear about it in the readme or other communication with users.

[dzbarsky]

note that this is a subtle compatibility break with rules_python, at least last time I dug into this code, rules_python users couldn't adopt rules_py without changing their code.

Perhaps we can make the case that we no longer aim for drop-in compatibility with rules_python, but if we give that up we ought to be clear about it in the readme or other communication with users.

This is true; rules_python has already swapped determine_main to be a function instead of a standalone target, so let's at least do that if we don't want to make the bigger change. (Though I do think the new semantics are clearer)

[arrdem]

Gonna just redo this by vendoring the determine_main code and keeping compat for now.