Act as a replacement for .venv/bin/python

[brettcannon]

Basically if the launcher is named python and has a ../pyvenv.cfg, make it so it acts like a virtual environment. Would make creating virtual environments a bit lighter.

• VENV_REDIRECT variant in launcher.c, __PYVENV_LAUNCHER__ functionality of the macOS framework build launcher
• Might need switch off CLI additions -- i.e. -h, --list, and version specifier support -- in this situation to make this work
• Read ../pyvenv.cfg and its home and version keys to determine the Python executable to run
• Acts as a heavyweight "symlink" to the Python executable for the virtual environment
• Speeds up environment creation by not having to copy over entire Python installation on Windows (e.g. .pyd files)
• See/edit the site module to gain ability to specify virtual environment location (while maintaining the invariant on how to detect virtual environments as outlined in the venv module docs); probably making __PYVENV_LAUNCHER__ not be macOS-specific

Would need to verify that this is actually worth it on Windows before pursuing the work.