Import fix for namespaced packages

Summary: This diff is basically just an import of pex-tool/pex#338, but without the tests (we don't have the pex tests in this repo?). This solves a problem where the pex would fail to run any time you had a namespaced package in your path.

Test Plan: eyes

Reviewed By: sdwilsh

fbshipit-source-id: 8d500c3

third-party/py/pex/README.facebook

@@ -11,3 +11,4 @@ Local modifications:
  - Added the ability to not hash the contents of the pex (used for in-place builds).
  - Added support for directory resources
  - Back-ported Python 3.6 compatibility commit c5ab73fd4d8e816e21a89d48c8d0c8095ef5a49c
+ - Back-ported namespaced packages fix, commit 7d2dc7f500aa7ae227c3ddca4b278b807d353a5e

third-party/py/pex/pex/pex.py

@@ -140,6 +140,11 @@ def minimum_sys_modules(cls, site_libs, modules=None):
         new_modules[module_name] = module
         continue
 
+      # Unexpected objects, e.g. namespace packages, should just be dropped:
+      if not isinstance(module.__path__, list):
+        TRACER.log('Dropping %s' % (module_name,), V=3)
+        continue
+
       # Pop off site-impacting __path__ elements in-place.
       for k in reversed(range(len(module.__path__))):
         if cls._tainted_path(module.__path__[k], site_libs):