registry: fix non-unique default dataSources

Fix the problem where `registry.defaultDataSources` has two instances:

 - `require('loopback').defaultDataSources` used by
   `loopback.autoAttach()`

 - `require('./registry').defaultDataSources` used by
   `app.dataSource`.

I am intentionally leaving out unit-tests as the whole `autoAttach`
feature is going to be deleted before 2.0 is released.