Per-Version Index Cache in ~/.cabal directory

Switching between cabal install versions can occur either when developing cabal, or when using nix-shells that bring different versions into scope, among other reasons.

Each time you switch versions, you get `Warning: Parsing the index cache failed` and it needs to regenerate the index cache.

This can be tedious and time consuming.

I suggest that cabal store its index cache per-cabal-version rather than always in the same spot, thus helping to more frequently avoid this issue.

Start by tracing how cabal-install reads and writes the index cache under ~/.cabal, including how the active Cabal version can be identified. Define completion as keeping caches separate across Cabal versions and avoiding the parsing warning and regeneration when switching between them; verify the behavior by switching versions or environments.

Written by the indexing model from the issue text.