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>

net_util/src/open_tap.rs

@@ -2,12 +2,14 @@
 //
 // SPDX-License-Identifier: Apache-2.0 AND BSD-3-Clause
 
-use super::{vnet_hdr_len, MacAddr, Tap, TapError};
 use std::net::Ipv4Addr;
 use std::path::Path;
 use std::{fs, io};
+
 use thiserror::Error;
 
+use super::{vnet_hdr_len, MacAddr, Tap, TapError};
+
 #[derive(Error, Debug)]
 pub enum Error {
     #[error("Failed to convert an hexadecimal string into an integer: {0}")]