[FIX] module finder path parameter should be optional

According to PEP302, the signature of `Finder.find_module` should be 
`find_module(fullname, path=None)`.

Ever since it was introduced in 64ec5f3 the addons import hook 
defines the second parameter as mandatory, which is an issue for
systems relying on the specified behaviour (and not needing to
provide a path) like the stdlib's `pkgutil.find_loader`.

fixes odoo#10670

openerp/modules/module.py

@@ -65,7 +65,7 @@ class AddonsImportHook(object):
     thus `import openerp.addons.module`.
     """
 
-    def find_module(self, module_name, package_path):
+    def find_module(self, module_name, package_path=None):
         module_parts = module_name.split('.')
         if len(module_parts) == 3 and module_name.startswith('openerp.addons.'):
             return self # We act as a loader too.