Fix when packages are unvendored

When working these changes back upstream to pip, we realized that the
previous fix wasn't ideal since unvendoring the packages broke the
imports. For example, if urllib3 were unvendored, then the following
would fail:

    from requests.packages import urllib3

requests/packages/__init__.py

@@ -27,10 +27,13 @@ import sys
 
 class VendorAlias(object):
 
-    def __init__(self, package_name):
-        self._package_name = package_name
+    def __init__(self, package_names):
+        self._package_names = package_names
         self._vendor_name = __name__
-        self._vendor_pkg = self._vendor_name + "." + self._package_name
+        self._vendor_pkg = self._vendor_name + "."
+        self._vendor_pkgs = [
+            self._vendor_pkg + name for name in self._package_names
+        ]
 
     def find_module(self, fullname, path=None):
         if fullname.startswith(self._vendor_pkg):
@@ -45,6 +48,14 @@ class VendorAlias(object):
                 )
             )
 
+        if not (name == self._vendor_name or
+                any(name.startswith(pkg) for pkg in self._vendor_pkgs)):
+            raise ImportError(
+                "Cannot import %s, must be one of %s." % (
+                    name, self._vendor_pkgs
+                )
+            )
+
         # Check to see if we already have this item in sys.modules, if we do
         # then simply return that.
         if name in sys.modules:
@@ -93,4 +104,4 @@ class VendorAlias(object):
         return module
 
 
-sys.meta_path.extend([VendorAlias("urllib3"), VendorAlias("chardet")])
+sys.meta_path.append(VendorAlias(["urllib3", "chardet"]))