<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def lookup(self, module_name):
 """Searches for a file providing given module. Returns the normalized module id and path of the file. """

        for search_path in self._paths:
            module_path = os.path.join(search_path, module_name)
            new_module_name, module_file = self._lookup(module_path, module_name)
            if module_file:
                return new_module_name, module_file
        return None, None