Make cache more agnostic to venvs

I noticed that the mypy cache will mark almost everything as stale if the cache was produced with a venv different from the one running mypy. I think this is due to a full path being placed into cache files & interface_hashes, for files that are stdlib or packages. Then those packages, and everything depending on them, gets invalidated (which ends up being 99% of everything in big projects). I think it would be cool if the cache were more agnostic to this, for two reasons:

1. when testing locally, I occasionally use a second venv with maybe 1 or 2 different packages to test out changes (maybe a package upgraded or added types). Ideally only those packages and their descendants would be invalidated.
2. when trying out remote caches, the CI venv will be "different" (existing at a different path), but the lockfile will be exactly the same. Ideally nothing would be invalidated.

Commencez par suivre comment mypy écrit et compare les fichiers de cache et les interface_hashes, en particulier là où sont enregistrés les chemins complets des environnements virtuels. Reproduisez l’invalidation avec deux chemins de venv et comparez le comportement du cache. Le travail est terminé lorsque des environnements équivalents avec des chemins différents évitent une invalidation globale, tandis que les paquets modifiés et leurs descendants deviennent toujours obsolètes.