Load upgrade modules.

        :param remove_applied: if True, already applied upgrades will not
            be included, if False the entire upgrade graph will be
            returned.

def _load_upgrades(self, remove_applied=True):
        """Load upgrade modules.

        :param remove_applied: if True, already applied upgrades will not
            be included, if False the entire upgrade graph will be
            returned.
        """
        if remove_applied:
            self.load_history()

        for entry_point in iter_entry_points('invenio_upgrader.upgrades'):
            upgrade = entry_point.load()()
            self.__class__._upgrades[upgrade.name] = upgrade

        return self.__class__._upgrades