ng generate component in library doesn't update public_api and exports

### Bug Report or Feature Request (mark with an `x`)
```
- [ ] bug report -> please search issues before submitting
- [x ] feature request
```

### Command (mark with an `x`)
```
- [ ] new
- [ ] build
- [ ] serve
- [ ] test
- [ ] e2e
- [x ] generate
- [ ] add
- [ ] update
- [ ] lint
- [ ] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc
```

### Versions:
```
Angular CLI: 7.2.1
Node: 8.11.3
OS: win32 x64
Angular: 7.2.0
```

After generating `library`, I need to add a component to my library by following:
```
ng generate component foo --project=foo-lib
```

It doesn't update `public_api.ts` and `exports` in `my-lib.module.ts` file. These steps must be done by hand.

首先，在报告的 Angular CLI 7.2.1 设置下重现 `ng generate component foo --project=foo-lib`，然后检查生成的 `public_api.ts` 和 `my-lib.module.ts` 文件。当组件生成能够在无需手动编辑的情况下更新库的公共 API 和模块导出时，此更改即完成。

由索引模型根据 Issue 内容生成。