lb4 openapi doesn't work with openapi specs generated by loopback itself

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### Describe the bug

The `lb4 openapi` command fails to generate working code in case the specification provided is generated by loopback itself. This shouldn't be the case as the specification is generated by LoopBack and the framework uses that same spec.

To reproduce this, generate specs from any lb4-based project. Then use that same specification to generate openapi artifacts with lb4 `openapi`. The resulting app creates buggy code.

The reproduction app is built based on openapi specs of [this](https://github.com/aaqilniz/lb4-ex-ref-many) application.

### Logs

```shell
src/models/index.ts:2:1 - error TS2308: Module './account-with-relations.model' has already exported a member named 'AccountWithRelations'. Consider explicitly re-exporting to resolve the ambiguity.

2 export * from './account.model';
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/models/index.ts:6:1 - error TS2308: Module './customer-with-relations.model' has already exported a member named 'CustomerWithRelations'. Consider explicitly re-exporting to resolve the ambiguity.

6 export * from './customer.model';
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

### Additional information

I tried removing duplicate entries which have been causing the problems. The resulting code still was not working and causing another issue. The request passed to swagger-client had `filter=undefined` in the URL which was making swagger-client fail to fetch any data.

### Reproduction

https://github.com/aaqilniz/lb4-ex-openapi

Reproduisez l’échec avec l’application lb4-ex-openapi en utilisant une spécification générée par l’application LoopBack liée. Commencez par le flux de génération OpenAPI de lb4 et examinez les exports de src/models/index.ts généré ainsi que la requête transmise à swagger-client. C’est terminé lorsque l’application générée se compile sans exports de modèles en double et que les requêtes ne contiennent pas filter=undefined.

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