Issue #19728: add private ensurepip._uninstall CLI

MvL would like to be able to preserve CPython's existing clean
uninstall behaviour on Windows before enabling the pip
installation option by default.

This private CLI means running "python -m ensurepip._uninstall"
will remove pip and setuptools before proceeding with the rest
of the uninstallation process.

If the version of pip differs from the one bootstrapped by
CPython, then the uninstallation helper will leave it alone
(just like any other pip installed packages)