Register imported modules and usage symbols.

def visit_ImportFrom(self, node):
        """ Register imported modules and usage symbols.  """
        module_path = tuple(node.module.split('.'))
        self.imports.add(module_path[0])
        for alias in node.names:
            path = module_path + (alias.name,)
            self.symbols[alias.asname or alias.name] = path
        self.update = True
        return None