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.

Start with the generate command and inspect the existing library schematic structure. Review test.ts and angular.json, especially the context path and sourceRoot settings described in the issue. Done means a schematic can generate a library secondary entry point with the needed testing and coverage configuration.

Written by the indexing model from the issue text.