Import distutils only if needed, but sooner

[sbidoul]

fixes #11319

[pradyunsg]

We did it too well. 😆

We should really add tests for this mess.

[uranusjr]

I wonder how we can test this; just try to do pip install -U in a temporary virtual environment?

[sbidoul]

This only happens in user site because in uninstall we have

pip/src/pip/_internal/req/req_uninstall.py

Lines 554 to 557 in 74f850d

	if dist.in_usersite:
	bin_dir = get_bin_user()
	else:
	bin_dir = get_bin_prefix()

and get_bin_user() does not import _distutils (so it is not in memory when installing later) while get_bin_prefix() does.

[sbidoul]

To test this we need to install pip in user site, then force-reinstall it from a wheel. I've no idea how to do that in our test suite.