This is intended to help prevent the following scenario from happening:

1. Old cargo builds an index cache.
2. Old cargo finds an index entry it cannot parse, skipping it,
   and saving the cache without the entry.
3. New cargo loads the cache with the missing entry, and never sees
   the new entries that it understands.

This may result in more cache thrashing, but that seems better than
having new cargos missing entries.

The whole point is that when loading a cache from an earlier version,
that cache may be missing new entries. So don't load older caches.