Auto merge of #4267 - natboehm:add-index-flag-search, r=matklad

Replace `cargo search --host` with `cargo search --index`

This change makes the command consistent with other versions of the same flag. `cargo search --host` is still supported, as well as `cargo search --index <index> --host <host>` in case of confusion. The three commands currently supported should produce the same behavior. All instances of `--host` are currently marked as deprecated, or for reuse as mentioned in issue #4208.

Fixes #4219