Copy VendorImporter. Ref #1296.

setuptools/extern/__init__.py

-from pkg_resources.extern import VendorImporter
+import sys
+
+
+class VendorImporter:
+    """
+    A PEP 302 meta path importer for finding optionally-vendored
+    or otherwise naturally-installed packages from root_name.
+    """
+
+    def __init__(self, root_name, vendored_names=(), vendor_pkg=None):
+        self.root_name = root_name
+        self.vendored_names = set(vendored_names)
+        self.vendor_pkg = vendor_pkg or root_name.replace('extern', '_vendor')
+
+    @property
+    def search_path(self):
+        """
+        Search first the vendor package then as a natural package.
+        """
+        yield self.vendor_pkg + '.'
+        yield ''
+
+    def find_module(self, fullname, path=None):
+        """
+        Return self when fullname starts with root_name and the
+        target module is one vendored through this importer.
+        """
+        root, base, target = fullname.partition(self.root_name + '.')
+        if root:
+            return
+        if not any(map(target.startswith, self.vendored_names)):
+            return
+        return self
+
+    def load_module(self, fullname):
+        """
+        Iterate over the search path to locate and load fullname.
+        """
+        root, base, target = fullname.partition(self.root_name + '.')
+        for prefix in self.search_path:
+            try:
+                extant = prefix + target
+                __import__(extant)
+                mod = sys.modules[extant]
+                sys.modules[fullname] = mod
+                # mysterious hack:
+                # Remove the reference to the extant package/module
+                # on later Python versions to cause relative imports
+                # in the vendor package to resolve the same modules
+                # as those going through this importer.
+                if sys.version_info > (3, 3):
+                    del sys.modules[extant]
+                return mod
+            except ImportError:
+                pass
+        else:
+            raise ImportError(
+                "The '{target}' package is required; "
+                "normally this is bundled with this package so if you get "
+                "this warning, consult the packager of your "
+                "distribution.".format(**locals())
+            )
+
+    def install(self):
+        """
+        Install this importer into sys.meta_path if not already present.
+        """
+        if self not in sys.meta_path:
+            sys.meta_path.append(self)
+
 
 names = 'six',
 VendorImporter(__name__, names, 'pkg_resources._vendor').install()