Collect the type aliases in the source.

        :sig: () -> None

def collect_aliases(self):
        """Collect the type aliases in the source.

        :sig: () -> None
        """
        self.aliases = get_aliases(self._code_lines)
        for alias, signature in self.aliases.items():
            _, _, requires = parse_signature(signature)
            self.required_types |= requires
            self.defined_types |= {alias}