Library secondary entry-point generation

# 🚀 Feature request

### Command (mark with an `x`)

- [ ] new
- [ ] build
- [ ] serve
- [ ] test
- [ ] e2e
- [x] generate
- [ ] add
- [ ] update
- [ ] lint
- [ ] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

### Description

Creating a secondary entry points for libraries is a manual effort with a risk of misconfiguration of unit testing environment.
In order to get a proper coverage report for all the libraries, the following needs to be adjusted:

- in `test.ts` the context path should be changed: `const context = require.context('../', true, /\.spec\.ts$/);`
- in `angular.json` the `sourceRoot` path should be changed: `"sourceRoot": "projects/lib-name",` (no `/src`)

### Describe the solution you'd like

Create a schematic that generates the secondary entry point for the given library.

---

I would be happy to provide a PR if this get's greenlit.

Commencez par la commande generate et examinez la structure existante du schematic de library. Examinez test.ts et angular.json, en particulier les paramètres de context path et sourceRoot décrits dans l’issue. Le travail est terminé lorsqu’un schematic peut générer un point d’entrée secondaire d’une library avec la configuration nécessaire pour les tests et la couverture.

Rédigé par le modèle d'indexation à partir du texte de l'issue.