pex shadows pkg_resources, which can break newer versions of setuptools

[kwlzn]

with a version of setuptools inside pex's allowed range:

[omerta pants (kwlzn/pantsd_bugfixes)]$ pex "setuptools<34"
Python 2.7.10 (default, Dec 16 2015, 14:09:45) 
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from setuptools.sandbox import run_setup
>>> 

with a newer one:

[omerta pants (kwlzn/pantsd_bugfixes)]$ pex setuptools==36.2.7
Python 2.7.10 (default, Dec 16 2015, 14:09:45) 
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from setuptools.sandbox import run_setup
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/private/var/folders/bb/msvj_4_s741g93nzgrj19twc0000gn/T/tmp1zkDfP/.deps/setuptools-36.2.7-py2.py3-none-any.whl/setuptools/sandbox.py", line 15, in <module>
    import pkg_resources.py31compat
ImportError: No module named py31compat
>>> 

because:

>>> import pkg_resources
>>> pkg_resources.__file__
'/private/var/folders/bb/msvj_4_s741g93nzgrj19twc0000gn/T/tmpsTSy8w/.bootstrap/pkg_resources/__init__.pyc'
>>> 

poc:

[omerta pants (kwlzn/pantsd_bugfixes)]$ pex setuptools==36.2.7
Python 2.7.10 (default, Dec 16 2015, 14:09:45) 
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> import sys
>>> sys.path
['/private/var/folders/bb/msvj_4_s741g93nzgrj19twc0000gn/T/tmpCoO7yi/.bootstrap', '/var/folders/bb/msvj_4_s741g93nzgrj19twc0000gn/T/tmpCoO7yi', '/opt/ee/python/2.7/lib/python27.zip', '/opt/ee/python/2.7/lib/python2.7', '/opt/ee/python/2.7/lib/python2.7/plat-darwin', '/opt/ee/python/2.7/lib/python2.7/plat-mac', '/opt/ee/python/2.7/lib/python2.7/plat-mac/lib-scriptpackages', '/opt/ee/python/2.7/lib/python2.7/lib-tk', '/opt/ee/python/2.7/lib/python2.7/lib-old', '/opt/ee/python/2.7/lib/python2.7/lib-dynload', '/private/var/folders/bb/msvj_4_s741g93nzgrj19twc0000gn/T/tmpCoO7yi/.deps/setuptools-36.2.7-py2.py3-none-any.whl']
>>> sys.path.insert(0, sys.path[-1])
>>> del sys.modules['pkg_resources']
>>> from setuptools.sandbox import run_setup
>>> 

[jsirois]

We probably need to vendor our .bootstrap/ code up to and including pex itself on the off chance a user wants to create a pex of their own code that uses pex as a library.

[jsirois]

Or... delegate to the final picked runner in a contextmanager that removes .bootstrap from the sys.path and the re-adds it on exit so pex exit handlers - if any can run - which is the POC above. I'll try this first.