Improve size measurement

- [ ] Instead of just running `du -sh .` in the package directory (which only works if installing the package stores everything locally), get a measure of how much space was used pre/post installation.
- [ ] When installing a set of packages, sum the estimated size increase rather than just including them on a per-package basis.

No files or tests are named. Locate the package-installation entry point and inspect where `du -sh .` is currently run; compare filesystem usage before and after installation, then check the path that handles sets of packages. Done means reporting the installation size increase and summing the estimated increase for a package set rather than counting each package independently.

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