pkg_resources should not depend on setuptools

Originally reported by: jaraco (Bitbucket: jaraco, GitHub: jaraco)

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With the 8.0 release, and the vendoring of packaging, pkg_resources now imports setuptools, invoking the side effects of importing setuptools. This change violates the implicit expectation that pkg_resources should run independent of setuptools. I missed this violation during my review, but it should be relatively straightforward to correct.

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/311

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Include api_tests.txt in pytest invocation. Ref #312. Ref #311.