Local no-index repositories cache leads to bad UX

**Describe the bug**

It's [documented](https://cabal.readthedocs.io/en/stable/config.html#local-no-index-repositories) that, when using local no-index repositories.

> cabal-install creates a .cache file, and will aggressively use its contents if it exists. Therefore if you change the contents of the directory, remember to wipe the cache too.

but cabal-install offers no warning or message about this leaving the user staring, confused, at a solver failure. See for example #9133.

**Expected behavior**

Cabal should either

1. Provide a better caching mechanism, detecting changes to the local repository.
2. Remnind the user that it will only look at the cache file and that the user should manually delete the cache file if the repository has changed.