[darwinbuild] xar subdoc usage

I've made a first pass at trying to include some of the darwinbuild related metadata in the xar archives created by packageRoots. Basically, I've added a darwinxref module that just takes the project information that is in the plist from the database and exports it in either plist or xml style like exportIndex does. Then packageRoots calls this, exports the information in xml format, and has xar add it as a subdoc in the archive named 'darwinbuild'. Rob