Provide a way to autodetect the Python runtime in pip_import

[brandjon]

(Forked from discussion in #256.)

When no python_interpreter is specified for pip_import, it defaults to "python". This command is usually the system Python 2 interpreter, but on some platforms (Arch, I believe) it can be Python 3.

"python2" is more specific, but it doesn't exist on all platforms that have Python 2 (macOS in particular: see bazelbuild/bazel#9402).

It'd be nice to autodetect what platform we're on and choose the right Python 2 command for pip_import by default. Note that this is the WORKSPACE equivalent of an autodetecting Python toolchain, and may have some overlap with the aim of #257.

[github-actions]

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[github-actions]

This issue was automatically closed because it went 30 days without a reply since it was labeled "Can Close?"