Register imported modules and usage symbols.

def visit_Import(self, node):
        """ Register imported modules and usage symbols.  """
        for alias in node.names:
            alias_name = tuple(alias.name.split('.'))
            self.imports.add(alias_name[0])
            if alias.asname:
                self.symbols[alias.asname] = alias_name
            else:
                self.symbols[alias_name[0]] = alias_name[:1]
            self.update = True
        return None