Extract function for _clear_modules, encapsulating the need for the module names to be greedily evaluated before removing them.

Setuptools now uses the 'packaging' package from pkg_resources, unifying the behavior around resolution of that package.

Move vendored packaging module into pkg_resources._vendor, restoring independence of pkg_resources from setuptools. Fixes #311.

Add test making explicit the expectation that pkg_resources shouldn't import setuptools (or be dependent on it in any way). Ref #311.