When an exact version is being installed, if we already have that
version from the index, we don't need to update the index before
installing it. Don't do this if it's not an exact version, because the
update may find us a newer version.

This is particularly useful for scripts which unconditionally run
`cargo install some-crate --version=1.2.3`. Before install-update, I
wrote a crate to do this
(https://crates.io/crates/cargo-ensure-installed) which I'm trying to
replace with just `cargo install`, but the extra latency of updating the
index for a no-op is noticeable.

This introduces an interesting edge-case around yanked crates; the
yanked-ness of crates will no longer change on install for exact version
matches which were already downloaded. This feels niche enough to
hopefully not matter...

Handling of these is coupled, so do the handling in one place, close to
where we parse the command line flags, so we can just pass in a single
derived object.

It only ever actually uses one, so let's reflect that in the types.

Remove enum, pull out function and dependent function

semver hasn't merged the upstream PR (yet)