feat: add autodetecting toolchain compatible with rules_py venvs

[mattem]

Fixes #21

$ which python3
/opt/homebrew/bin/python3

$ python3 --version
Python 3.9.10

$ bazel run --override_repository rules_py=/Users/matt/Documents/workspace/rules_py ipython
INFO: Analyzed target //:ipython (26 packages loaded, 95 targets configured).
INFO: Found 1 target...
Target //:ipython up-to-date:
  bazel-bin/ipython
INFO: Elapsed time: 1.458s, Critical Path: 1.33s
INFO: 3 processes: 2 internal, 1 darwin-sandbox.
INFO: Build completed successfully, 3 total actions
INFO: Build completed successfully, 3 total actions

Python: /private/var/tmp/_bazel_matt/b28310042610550fd4cf1ffc282c1f7a/execroot/rules_py_example/bazel-out/darwin_arm64-fastbuild/bin/ipython.runfiles/ipython.venv/bin/python_real
version: 3.9.10 (main, Jan 15 2022, 11:40:53)
[Clang 13.0.0 (clang-1300.0.29.3)]
version info: sys.version_info(major=3, minor=9, micro=10, releaselevel='final', serial=0)
cwd: /private/var/tmp/_bazel_matt/b28310042610550fd4cf1ffc282c1f7a/execroot/rules_py_example/bazel-out/darwin_arm64-fastbuild/bin/ipython.runfiles/rules_py_example
site-packages folder: ['/private/var/tmp/_bazel_matt/b28310042610550fd4cf1ffc282c1f7a/execroot/rules_py_example/bazel-out/darwin_arm64-fastbuild/bin/ipython.runfiles/ipython.venv/lib/python3.9/site-packages']
VIRTUAL_ENV=/private/var/tmp/_bazel_matt/b28310042610550fd4cf1ffc282c1f7a/execroot/rules_py_example/bazel-out/darwin_arm64-fastbuild/bin/ipython.runfiles/ipython.venv
Python 3.9.10 (main, Jan 15 2022, 11:40:53)
Type 'copyright', 'credits' or 'license' for more information
IPython 8.3.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]:

[alexeagle]

Does this necessarily belong in rules_py? Should rules_python have such a thing?

[alexeagle]

What does auto detecting mean? Should point out its non hermetic but works better with c/c++ wheel builds

[mattem]

Does this necessarily belong in rules_py? Should rules_python have such a thing?

I don't think this should be part of rules_python, the venv handling is specific to rules_py.