Find any valid target for "source()" in the graph by its name and
        package name, or None for any package.

def find_source_by_name(self, source_name, table_name, package):
        """Find any valid target for "source()" in the graph by its name and
        package name, or None for any package.
        """
        name = '{}.{}'.format(source_name, table_name)
        return self._find_by_name(name, package, 'nodes', [NodeType.Source])