Installation issues when previously-imported pkg_resources are present

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

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Using a custom implementation of bootstrap-buildout.py, which is very similar, installing setuptools gives the following error:

Downloading https://pypi.python.org/packages/source/s/setuptools/setuptools-18.4.zip
Extracting in /var/folders/4w/837bbxsj6ls8jkczsp029jx40000gp/T/tmp4ha6th
Now working in /var/folders/4w/837bbxsj6ls8jkczsp029jx40000gp/T/tmp4ha6th/setuptools-18.4
Building a Setuptools egg in /var/folders/4w/837bbxsj6ls8jkczsp029jx40000gp/T/tmpyf5BFP
/var/folders/4w/837bbxsj6ls8jkczsp029jx40000gp/T/tmpyf5BFP/setuptools-18.4-py2.7.egg
Traceback (most recent call last):
  File "bootstrap.py", line 82, in <module>
    ez['use_setuptools'](**setup_args)
  File "<string>", line 176, in use_setuptools
  File "<string>", line 131, in _do_download
  File "build/bdist.macosx-10.10-x86_64/egg/setuptools/__init__.py", line 12, in <module>
  File "build/bdist.macosx-10.10-x86_64/egg/setuptools/extension.py", line 8, in <module>
  File "build/bdist.macosx-10.10-x86_64/egg/setuptools/dist.py", line 19, in <module>
  File "build/bdist.macosx-10.10-x86_64/egg/pkg_resources/__init__.py", line 90, in <module>
AttributeError: 'module' object has no attribute '_vendor'

This issue seems very similar to https://bitbucket.org/pypa/setuptools/issues/345, which seems to have been fixed in version 12.2, but seems to have been reintroduced in a later version.

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/453

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

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I have already figured out a fix to this issue, which is the same as the fix for https://bitbucket.org/pypa/setuptools/issues/345, and have it tested on my local machine. I am currently in the process of creating a pull request.

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

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Added pull request: https://bitbucket.org/pypa/setuptools/pull-requests/153/unload-all-pkg_resources-modules-and-not/diff that fixes this issue. Tested locally on version 18.4.

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

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Unload all pkg_resources modules and not just the main module. Fixes #453.