Auto merge of #9476 - ehuss:index-cache-bump, r=alexcrichton

Bump index cache version to deal with semver metadata version mismatch.

#9467 has uncovered an issue with how versions are handled in the index cache.  When using a debug build of Cargo, it may panic due to the [cache contents changing](https://github.com/rust-lang/cargo/blob/5c455130b6001c7f54e872e161c27f6e996aff1f/src/cargo/sources/registry/index.rs#L606-L619).  The particular problem I am running into is that the index has an entry for `openssl-src 110.0.0` and `110.0.0+1.1.0f`. This is due to an issue with crates.io where it allows publishing multiple versions with the same metadata (https://github.com/rust-lang/crates.io/issues/1059).  Cargos before #9467 would populate the index cache with two entries, both with version `110.0.0`.  Afterwards, there are two separate entries (`110.0.0` and `110.0.0+1.1.0f`).

The change here is to bump the index cache version so that new versions of cargo will clear the cache, and won't trigger the assertion.

This may be a bit of a heavy-handed solution, as I think this only affects debug builds of cargo.  However, I instantly started running into this problem, so I suspect it will be a real annoyance for anyone developing cargo. Happy to discuss other possible solutions.