misc: Adapt consistent import style formatting

Historically the Cloud Hypervisor coding style has been to ensure that
all imports are ordered and placed in a single group. Unfortunately
cargo fmt has no support for ensuring that all imports are in a single
group so if whitespace lines were added as part of the import statements
then they would only be odered correctly in the group.

By adopting "group_imports="StdExternalCrate" we can enforce a style
where imports are placed in at most three groups for std, external
crates and the crate itself. Choosing a style enforceable by the tooling
reduces the reviewer burden.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

tracer/src/tracer.rs

@@ -3,8 +3,6 @@
 // SPDX-License-Identifier: Apache-2.0
 //
 
-use once_cell::unsync::OnceCell;
-use serde::Serialize;
 use std::collections::HashMap;
 use std::fs::File;
 use std::io::Write;
@@ -12,6 +10,9 @@ use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::{Arc, Mutex};
 use std::time::{Duration, Instant};
 
+use once_cell::unsync::OnceCell;
+use serde::Serialize;
+
 #[derive(Debug)]
 struct Tracer {
     events: Arc<Mutex<HashMap<String, Vec<TraceEvent>>>>,