Allow to extend ProjectBrowserSource plugins

I'm speaking a bit outside my comfort zone here, but I know that we have used a Decorator pattern in order to create the "Local modules" Source plugin.

I don't know much about decorators, so I am just coming to ask the question - whether or not approaching it from that paradigm solves the issues or if we would still need to have these methods be protected instead of private?

That aside, I think @penyaskito's comment is actually what the intention was here, to bring in /recipes & /core/recipes together - but we were waiting on recipes being more stable to do so, or have some discoverability from d.o... though maybe that should have been marked with a @todo.