When package does not specify its own tranform, use transform from opts

Ran into this behavior in https://github.com/substack/node-browserify/issues/501.  Right now, non-toplevel modules cannot be transformed unless they specify transforms in their own package.json.  

IMO, I would expect transforms provided by client code (e.g. the `opts.transform` provided by browserify) to be the default fallback for non-configured modules, rather than falling back to no transforms at all.  Does this make sense?