setup: don't import distutils prior to checking FORCE_SETUPTOOLS
I've seen warnings about importing distutils before setuptools,
although I can't reproduce them at the moment.
Differential Revision: https://phab.mercurial-scm.org/D9722
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.