extract-i18n does not extract message from files loaded only server side

# 🐞 Bug report

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

- [ ] new
- [ ] build
- [ ] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [ ] add
- [ ] update
- [ ] lint
- [x ] extract-i18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

### Is this a regression?

Yes, the previous version in which this bug was not present was: ....

### Description
The extraction tool does not process files that do not have a direct dependency on the app.module, or, generally, on the browser side.
In my case an abstract service is replaced by two versions of the service, one browser side, the other server side. The server-side one is ignored during the extraction process.

## 🔬 Minimal Reproduction

I made a simple [repro app](https://github.com/manzonif/ng-extract-i18n-server) that reproduce the problem.
Just run extract-i18n to see that message in [server-service.service.ts](https://github.com/manzonif/ng-extract-i18n-server/blob/master/src/app/server-service.service.ts) is ignored.
## 🔥 Exception or Error

## 🌍 Your Environment

Angular: 12.0.1

... animations, cdk, cli, common, compiler, compiler-cli, core

... forms, language-service, material, platform-browser

... platform-browser-dynamic, platform-server, router

... service-worker

Package                           Version

-----------------------------------------------------------

@angular-devkit/architect         0.1200.5

@angular-devkit/build-angular     12.0.5

@angular-devkit/build-optimizer   0.1200.5

@angular-devkit/core              12.0.1

@angular-devkit/schematics        12.0.1

@angular/flex-layout              12.0.0-beta.34

@angular/localize                 12.0.5

@nguniversal/builders             12.1.3

@nguniversal/common               12.0.0

@nguniversal/express-engine       12.0.0

@schematics/angular               12.0.1

rxjs                              7.4.0

typescript                        4.2.4

**Anything else relevant?**

Run extract-i18n in the linked repro app and compare its output with the message in server-service.service.ts. Trace how the command discovers files that are not directly dependent on app.module or the browser-side code. Done means messages from the server-side service are included in extraction output.

Scritto dal modello di indicizzazione a partire dal testo della issue.