Only store compressed index, don't store 01-index.tar

EDIT: Update 6 years later. Since version 2.0 cabal uses the new hackage-security protocol and stores the new 01-index but it does still store both the compressed and uncompressed version. As October 2023, the numbers are:

```
❯ ls -lh ~/.cabal/packages/hackage.haskell.org/01-index.tar*
-rw-r--r-- 1 andrea andrea 835M Oct 6 14:53 /home/andrea/.cabal/packages/hackage.haskell.org/01-index.tar
-rw-r--r-- 1 andrea andrea 112M Oct 6 14:14 /home/andrea/.cabal/packages/hackage.haskell.org/01-index.tar.gz
-rw-r--r-- 1 andrea andrea 5.0M Oct 6 14:14 /home/andrea/.cabal/packages/hackage.haskell.org/01-index.tar.idx
```

Original post follows:

My `packages/hackage.haskell.org/00-index.tar` consumes 200 MB and I wonder whether it is necessary to store it uncompressed. Cabal already stores `00-index.tar.gz`, too, which occupies only 10 MB. Is there a need to store the uncompressed archive on disk?

Start by tracing Cabal's handling of the Hackage index under ~/.cabal/packages/hackage.haskell.org/, including 01-index.tar, 01-index.tar.gz, and 01-index.tar.idx. Check the related index-storage tests, if present, and verify that package-index operations still work while the uncompressed archive is no longer retained.

Written by the indexing model from the issue text.