The vendor script should support skip markers.

[jsirois]

Right now, running tox -evendor modifies pex/environment.py:

$ git diff pex/environment.py
diff --git a/pex/environment.py b/pex/environment.py
index e161254..aff4d9e 100644
--- a/pex/environment.py
+++ b/pex/environment.py
@@ -32,12 +32,12 @@ from pex.util import CacheHelper, DistributionHelper
 
 def _import_pkg_resources():
   try:
-    import pkg_resources
+    import pex.third_party.pkg_resources as pkg_resources
     return pkg_resources, False
   except ImportError:
     from pex import third_party
     third_party.install(expose=['setuptools'])
-    import pkg_resources
+    import pex.third_party.pkg_resources as pkg_resources
     return pkg_resources, True

It should not touch these particular imports; so some mechanism to tell it to skip is needed. Something like a # vendor:skip should do.

[jsirois]

This is needed to support #650