Move distutils to setuptools._distutils

Ensure the module is named 'distutils'. Avoids errors when distutils.log and setuptools._distutils.log are two separate modules with separate state.

add pypy schemas to command/install.py

Add support for PyPy

In test_unixcompiler.test_osx*, also patch sysconfig.get_config_vars following the patterns of prior implementations.

Acknowledge and ignore warning about TestDistribution (it's a "test" distribution, not a "test of distributions").

Mark test_venv to be skipped when running under a virtualenv as virtualenv monkey patches distutils.

Replace distutils rather than requiring it to be present in advanec. Instead of crashing, issue a warning when Setuptools is replacing distutils.

Co-authored-by: Steve Dower <steve.dower@microsoft.com>

Defer finalize options

Avoid loading working set during `Distribution.finalize_options` prior to invoking `_install_setup_requires`, broken since v42.0.0. Fixes #2158.