ng add - to refer the add schematic from another npm module

### Observed behavior
Currently `ng add package-name` goes into `package-name` and looks for an `add` schematic.
I work on a schematic in @nativescript/schematics.
So, in order to run `ng add` for it, I need to call `ng add @nativescript/schematics`
While our main library is `nativescript`.

### Desired behavior
Ideally, I would like to be able to call `ng add nativescript`, however I can't move my schematics into `nativescript` repo at the moment.
Perhaps there could be a way to add a reference for @nativescript/schematics` to `nativescript`.

Start at the ng add command and trace how it resolves the add schematic from package-name. Compare the desired nativescript invocation with the current @nativescript/schematics lookup; done means a package can reference an add schematic supplied by another npm module.

索引モデルが issue の本文から書いたものです。