Use a per-version cache file for the index state

[mpickering]

cabal-install will now use a version suffixed cache file for the index state.

If you are regularly changing between cabal-install versions, this will be less annoying as you won't have to regenerate the cache each time you switch project.

There is one tricky part of the implementation. If you update the index with a newer cabal-install, then the old-style cabal-install caches are invalidated by replacing them with an empty file.
This is because in cabal-install (until this commit), the freshness of the cache was now checked by readIndexCache.

If you update with an older cabal-install then the freshness check will see the cache for your new cabal-install is older than the index, and update it.

Fixes #7502

Please read Github PR Conventions and then fill in one of these two templates.

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).