Alternative lookup convention for `ng-add` schematics

It would be nice to be able to externalize schematics from the package specified in the `ng add` command.

`ng add ionic` requires us to include schematics within the `ionic` package itself, which is not ideal for several reasons:
- **Separation of Concerns**. The `ionic` package isn't necessarily for Angular, but `ng add ionic` looks good 👌
- **Complication of the build within our main package.** Including the schematics in our main package would require including _a lot_ of the Angular DevKit into the build.
- **Slow "add".** It requires installing the entire package and all its dependencies (~38M) in a sandbox just to use a few files from the package.

In slack I believe we concluded that the best solution might be to have the `ng-add` field within `package.json` be allowed to point to another package.

Beginnen Sie mit dem Befehl `ng add` und dem Feld `ng-add` in `package.json`; prüfen Sie, wie das angegebene Paket derzeit für Schematics verwendet wird. Als abgeschlossen gilt die Änderung, wenn ein `ng-add`-Eintrag auf ein anderes Paket verweisen kann, sodass Schematics aus dem im Befehl genannten Paket ausgelagert werden können.

Vom Indexierungsmodell aus dem Issue-Text verfasst.