Update distutils patch to monkeypatch all paths from sys.path to ensure that...

Update distutils patch to monkeypatch all paths from sys.path to ensure that distutils is never imported except from the same path as setuptools. Assert that 'distutils' is not already in sys.modules.

setuptools/distutils_patch.py

@@ -7,9 +7,23 @@ for more motivation.
 
 import sys
 import importlib
+import contextlib
 from os.path import dirname
 
 
-sys.path.insert(0, dirname(dirname(__file__)))
-importlib.import_module('distutils')
-sys.path.pop(0)
+@contextlib.contextmanager
+def patch_sys_path():
+    orig = sys.path[:]
+    sys.path[:] = [dirname(dirname(__file__))]
+    try:
+        yield
+    finally:
+        sys.path[:] = orig
+
+
+if 'distutils' in sys.path:
+    raise RuntimeError("Distutils must not be imported before setuptools")
+
+
+with patch_sys_path():
+    importlib.import_module('distutils')