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>

performance-metrics/src/main.rs

@@ -8,9 +8,6 @@ extern crate test_infra;
 
 mod performance_tests;
 
-use clap::{Arg, ArgAction, Command as ClapCommand};
-use performance_tests::*;
-use serde::{Deserialize, Serialize};
 use std::{
     env, fmt,
     process::Command,
@@ -18,6 +15,10 @@ use std::{
     thread,
     time::Duration,
 };
+
+use clap::{Arg, ArgAction, Command as ClapCommand};
+use performance_tests::*;
+use serde::{Deserialize, Serialize};
 use test_infra::FioOps;
 use thiserror::Error;