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/tap.rs

@@ -5,19 +5,21 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the THIRD-PARTY file.
 
-use super::{
-    create_inet_socket, create_sockaddr, create_unix_socket, vnet_hdr_len, Error as NetUtilError,
-    MacAddr,
-};
-use crate::mac::MAC_ADDR_LEN;
 use std::fs::File;
 use std::io::{Error as IoError, Read, Result as IoResult, Write};
 use std::net;
 use std::os::raw::*;
 use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
+
 use thiserror::Error;
 use vmm_sys_util::ioctl::{ioctl_with_mut_ref, ioctl_with_ref, ioctl_with_val};
 
+use super::{
+    create_inet_socket, create_sockaddr, create_unix_socket, vnet_hdr_len, Error as NetUtilError,
+    MacAddr,
+};
+use crate::mac::MAC_ADDR_LEN;
+
 #[derive(Error, Debug)]
 pub enum Error {
     #[error("Couldn't open /dev/net/tun: {0}")]
@@ -428,7 +430,6 @@ mod tests {
     use std::time::Duration;
 
     use once_cell::sync::Lazy;
-
     use pnet::packet::ethernet::{EtherTypes, EthernetPacket, MutableEthernetPacket};
     use pnet::packet::ip::IpNextHeaderProtocols;
     use pnet::packet::ipv4::{Ipv4Packet, MutableIpv4Packet};