Add .cache entries for registry indices

[Jake Shadle]

Cargo has an optimization where it doesn't actually do a checkout of the registry, but it then needs to read the crate entries from the blob, which is a bit slower than just reading a file on disk, so it also generates .cache entries which it uses instead. I think this may be the missing piece for #16 for why doing a fetch immediately after using cargo-fetcher can still take multiple seconds, when it should actually be essentially a no-op.