Scoped modules

Partly a proof-of-concept of scoped modules in the couchapp registry for the purposes of dogfooding/bootstrapping, but more importantly, updates to the vdu so that users don't accidentally *depend on* scoped modules from existing global packages.

Review at your liesure: @seldo @othiym23 @iarna 
Should this actually be a restriction, I can see a world where a global module depends on, as an example:

`@browserify/jquery`