The only effect of the import is that ez_setup's use_setuptools() function
does a sys.exit(2) when the required version doesn't match the site package's
one.

If the import doesn't happen, use_setuptools() will just grab a matching
version if the site package's version is too old.

Note that the latest ez_setup.py scripts default to requiring the very latest
setuptools version! So --allow-site-packages isn't really very useful this
way.