Make VendorImporter more generic

--HG--
branch : feature/issue-229

pkg_resources/extern/__init__.py

 import sys
 
-_VENDORED_NAMES = 'packaging',
-_SEARCH_PATH = 'pkg_resources._vendor.', ''
-
 class VendorImporter:
     """
     A PEP 302 meta path importer for finding optionally-vendored
-    or otherwise naturally-installed packages from __name__.
+    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):
+        yield self.vendor_pkg + '.'
+        yield ''
+
     def find_module(self, fullname, path=None):
-        root, base, target = fullname.partition(__name__ + '.')
+        root, base, target = fullname.partition(self.root_name + '.')
         if root:
             return
-        if not any(map(target.startswith, _VENDORED_NAMES)):
+        if not any(map(target.startswith, self.vendored_names)):
             return
         return self
 
     def load_module(self, fullname):
-        root, base, target = fullname.partition(__name__ + '.')
-        for prefix in _SEARCH_PATH:
+        root, base, target = fullname.partition(self.root_name + '.')
+        for prefix in self.search_path:
             try:
                 __import__(prefix + target)
                 mod = sys.modules[prefix + target]
@@ -34,9 +41,9 @@ class VendorImporter:
                 "distribution.".format(**locals())
             )
 
-    @classmethod
-    def install(cls):
-        if not any(isinstance(imp, cls) for imp in sys.meta_path):
-            sys.meta_path.append(cls())
+    def install(self):
+        if self not in sys.meta_path:
+            sys.meta_path.append(self)
 
-VendorImporter.install()
+names = 'packaging',
+VendorImporter(__name__, names).install()

setuptools/extern/__init__.py

 import sys
 
-_VENDORED_NAMES = 'six',
-_SEARCH_PATH = 'setuptools._vendor.', ''
-
 class VendorImporter:
     """
     A PEP 302 meta path importer for finding optionally-vendored
-    or otherwise naturally-installed packages from __name__.
+    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):
+        yield self.vendor_pkg + '.'
+        yield ''
+
     def find_module(self, fullname, path=None):
-        root, base, target = fullname.partition(__name__ + '.')
+        root, base, target = fullname.partition(self.root_name + '.')
         if root:
             return
-        if not any(map(target.startswith, _VENDORED_NAMES)):
+        if not any(map(target.startswith, self.vendored_names)):
             return
         return self
 
     def load_module(self, fullname):
-        root, base, target = fullname.partition(__name__ + '.')
-        for prefix in _SEARCH_PATH:
+        root, base, target = fullname.partition(self.root_name + '.')
+        for prefix in self.search_path:
             try:
                 __import__(prefix + target)
                 mod = sys.modules[prefix + target]
@@ -34,9 +41,9 @@ class VendorImporter:
                 "distribution.".format(**locals())
             )
 
-    @classmethod
-    def install(cls):
-        if not any(isinstance(imp, cls) for imp in sys.meta_path):
-            sys.meta_path.append(cls())
+    def install(self):
+        if self not in sys.meta_path:
+            sys.meta_path.append(self)
 
-VendorImporter.install()
+names = 'six',
+VendorImporter(__name__, names).install()