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.

从 `ng add` 命令和 `package.json` 中的 `ng-add` 字段开始；检查当前如何将指定的包用于 schematics。完成标准是 `ng-add` 条目可以指向另一个包，从而允许将 schematics 从命令中指定的包外部化。

由索引模型根据 Issue 内容生成。