Support registration of JPEG plugins when the classloader can't find sun plugins

I'm trying to use the TwelveMonkeys JPEG plugins in an osgi bundle, but registration is failing because the classloader can't find the sun JPEG classes.  But as long as they have been registered we can still find them by iterating over all registered JPEG plugins.  It's definitely messier than using getServiceProviderByClass(), but it seems to be more general.