upgrade pex to latest version would break kolibri

[lyw07]

Observed behavior

In #5685, I tried to upgrade pex to latest version 1.6.7. The pex file was built successfully. However, when I run python kolibri.pex start, I got the error that stopped me from starting kolibri. My guess is that pex now vendors setuptools and wheel, and thus somehow pkg_resources cannot be imported from <pex_location>/.bootstrap. If we run pip install kolibri.whl, pkg_resources can be imported from the local setuptools library, since setuptools is usually installed with pip.

Expected behavior

kolibri pex should be run successfully

User-facing consequences

users cannot use pex file to run kolibri if we upgrade pex to latest version 1.6.7

Errors and logs

paths in the error log have been simplified for easier reading.

Traceback (most recent call last):
  File "kolibri.pex/.bootstrap/pex/pex.py", line 352, in execute
  File "kolibri.pex/.bootstrap/pex/pex.py", line 284, in _wrap_coverage
  File "kolibri.pex/.bootstrap/pex/pex.py", line 315, in _wrap_profiling
  File "kolibri.pex/.bootstrap/pex/pex.py", line 397, in _execute
  File "kolibri.pex/.bootstrap/pex/pex.py", line 495, in execute_entry
  File "kolibri.pex/.bootstrap/pex/pex.py", line 502, in execute_module
  File "/usr/local/Cellar/python@2/2.7.16/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 182, in run_module
    mod_name, loader, code, fname = _get_module_details(mod_name)
  File "/usr/local/Cellar/python@2/2.7.16/Frameworks/Python.framework/Versions/2.7/lib/python2.7/runpy.py", line 111, in _get_module_details
    __import__(mod_name)  # Do not catch exceptions initializing package
  File "kolibri/__init__.py", line 11, in <module>
    from .utils.version import get_version
  File "kolibri/utils/version.py", line 106, in <module>
    from .compat import parse_version
  File "kolibri/utils/compat.py", line 10, in <module>
    from pkg_resources import parse_version as _parse_version
ImportError: No module named pkg_resources

Steps to reproduce

get the pex from the PR linked in the description and run it.

Context