Construct arguments to use alternative package indexes if
        there were sources supplied, empty string if there were not.

def _get_install_sources(self):
        """Construct arguments to use alternative package indexes if
        there were sources supplied, empty string if there were not.
        """
        if not self._sources:
            return ''
        parts = ['--no-index']
        for source in self._sources:
            parts.append("--find-links '{0}'".format(source))
        return ' '.join(parts)