Implement autodetecting Python toolchain

This replaces the stub default Python toolchain with one that actually locates the target platform's Python interpreter at runtime. Try it out with

    bazel build //some_py_binary --experimental_use_python_toolchains

and note that, unlike before (#4815), the correct Python interpreter gets invoked by default regardless of whether you specify `--python_version=PY2` or `--python_version=PY3`.

This toolchain is only intended as a last resort, if the user doesn't define and register a better toolchain (that satisfies the target platform constraints).

Work toward #7375 and #4815. Follow-up work needed to add a test (#7843) and windows support (#7844).

RELNOTES: None
PiperOrigin-RevId: 240417315
7 files changed
tree: 65f79d1f67600072824b72b92abea10c15a6f963
  1. .bazelci/
  2. examples/
  3. scripts/
  4. site/
  5. src/
  6. third_party/
  7. tools/
  8. .gitattributes
  9. .gitignore
  10. AUTHORS
  11. BUILD
  12. CHANGELOG.md
  13. CODEOWNERS
  14. combine_distfiles.py
  15. combine_distfiles_to_tar.sh
  16. compile.sh
  17. CONTRIBUTING.md
  18. CONTRIBUTORS
  19. distdir.bzl
  20. ISSUE_TEMPLATE.md
  21. LICENSE
  22. README.md
  23. WORKSPACE
README.md

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