Bug ticket at the libraries repository: Zenject-usage.dll cannot be referenced in Unity 2020.1.2f1 if loaded as git package · Issue #165 · modesttree/Zenject · GitHub
In short:
Add to manifest
"com.svermeulen.extenject": "https://github.com/svermeulen/Extenject.git?path=UnityProject/Assets/Plugins/Zenject/#9.2.0"
- \PackageCache\com.svermeulen.extenject@c2e33500a8\Source\Usage\Zenject-usage.dll exists
- Zenject-usage.dll cannot be referenced