Let's not depend on a nightly pre-release, especially not just for a
linting option. We can enable the linter option once it lands in the
stable release.
By default, `rustfmt` persists the import granularity. In practice, most
Rust code has import granularity "Module" due to LSP's actions.
"Item" gets rid of import groupings and achieves cleaner diffs and
better conflict resolution. Better greppability is a positive side
effect.
Note: it's an unstable rustfmt feature. `cargo +nightly fmt` must be
used instead of `cargo fmt`.
Some developers might have interesting rust versions installed and set
as their default via `rustup default`, but we want to make sure that
everyone is using the same version as CI.
