extensions: use normpath to allow trailing '\' on Windows (issue4187)

Fixes same issue as 5c794e7331e7 but now works on Windows too.

With this patch a trailing backward slash won't prevent the extension from
being found on Windows, and we continue to support any combination of forward
and back slashes within the path.

mercurial/extensions.py

@@ -43,10 +43,10 b' def find(name):'
 
 def loadpath(path, module_name):
     module_name = module_name.replace('.', '_')
-    path = util.expandpath(path)
+    path = util.normpath(util.expandpath(path))
     if os.path.isdir(path):
         # module/__init__.py style
-        d, f = os.path.split(path.rstrip('/'))
+        d, f = os.path.split(path)
         fd, fpath, desc = imp.find_module(f, [d])
         return imp.load_module(module_name, fd, fpath, desc)
     else: