"Secure" cabal repositories use a new extended & incremental
`01-index.tar`. In order to avoid issues resulting from clobbering
new/old-style index data, we save them locally to different names.

With this patch, secure repos generate/update the files below on `cabal update`

- `01-index.cache`
- `01-index.tar`
- `01-index.tar.gz`
- `01-index.tar.idx`
- `mirrors.json`
- `root.json`
- `snapshot.json`
- `timestamp.json`

...while the legacy codepaths for non-secure repos operate on the files

- `00-index.cache`
- `00-index.tar`
- `00-index.tar.gz`
- `00-index.tar.gz.etag`

This way the old/new codepaths don't interfere with each other anymore.

Note: The format of `01-index.cache` file will be extended by the upcoming `--index-state` feature

This trivially fixes #3854