A catalog over a package.json checkout lists its packages again

A plugin catalog node that points at a local checkout of package.json packages, and declares no format, showed an empty catalog page after the recent change that made every reader of a catalog node agree on one format. The default those readers agreed on was the node-repo format, and nothing had ever needed to declare the manifest format before, so those catalogs went dark.

The catalog now reads the format off the checkout itself when none is declared: a folder of package.json packages with no node-repo roots is treated as a manifest catalog, and everything else keeps the shipped default. A format you declare on the node still wins.