Migrate a resource's subresources

    :param parent: the parent perch.Document instance
    :param migrations: the migrations for a resource

def _migrate_subresources(parent, migrations):
    """
    Migrate a resource's subresources

    :param parent: the parent perch.Document instance
    :param migrations: the migrations for a resource
    """
    for subresource, resource_migrations in migrations.items():
        parent = _migrate_subresource(
            subresource,
            parent,
            resource_migrations
        )

    return parent