Package considered zip_safe although package_data is used

Originally reported by: xflr6 (Bitbucket: xflr6, GitHub: xflr6)

The description of zip_safe inference in the docs:

it will consider the project unsafe if it contains any C extensions or datafiles whatsoever.

suggests that a package with a setup.py like this

from setuptools import setup, find_packages

setup(
    name='spam',
    version='0.1.dev0',
    packages=find_packages(),
    package_data={'spam': ['spam.txt']},
)

should be considered zip_safe=False.
However, when such a package is installed via python setup.py install, it ends up as a zipped egg:

copying build/lib.linux-i686-2.7/spam/spam.txt -> build/bdist.linux-i686/egg/spam
copying build/lib.linux-i686-2.7/spam/__init__.py -> build/bdist.linux-i686/egg/spam
(...)
Processing spam-0.1.dev0-py2.7.egg
Copying spam-0.1.dev0-py2.7.egg to /tmp/venv/lib/python2.7/site-packages
Adding spam 0.1.dev0 to easy-install.pth file

• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/386

Start with the zip_safe inference used by setup.py install, and reproduce the issue using the shown setup.py with package_data and spam.txt. Compare the behavior with the documentation's claim about data files; done means the inference and installed egg behavior agree with the documented expectation.