Inconsistent import order.

[stephennancekivell]

Hi, thanks for the project.

I noticed that when repeatedly running protoc with zap, the import order in the go code changes, despite nothing changing in the source proto files. It would be nice if it were always generating in the same order that gofmt sets.

For example with this small proto file.

main.proto

syntax = "proto3";

package test;

import "google/protobuf/struct.proto";
import "google/protobuf/timestamp.proto";

If I run it twice, I get a diff like this.

-	_ "google.golang.org/protobuf/types/known/structpb"
-	_ "google.golang.org/protobuf/types/known/timestamppb"
+	_ "google.golang.org/protobuf/types/known/timestamppb"
+	_ "google.golang.org/protobuf/types/known/structpb"

A full log is below.

$ protoc --zap-marshaler_out=. main.proto && cat main.zap.go 
main.proto:5:1: warning: Import google/protobuf/struct.proto is unused.
main.proto:6:1: warning: Import google/protobuf/timestamp.proto is unused.
// Code generated by protoc-gen-gogo. DO NOT EDIT.
// source: main.proto

package test

import (
	fmt "fmt"
	math "math"
	proto "github.com/golang/protobuf/proto"
	_ "google.golang.org/protobuf/types/known/timestamppb"      <<<<---- timestamp first
	_ "google.golang.org/protobuf/types/known/structpb"              <<<<---- structpb second
)

// Reference imports to suppress errors if they are not otherwise used.
var _ = proto.Marshal
var _ = fmt.Errorf
var _ = math.Inf

$ protoc --zap-marshaler_out=. main.proto && cat main.zap.go
main.proto:5:1: warning: Import google/protobuf/struct.proto is unused.
main.proto:6:1: warning: Import google/protobuf/timestamp.proto is unused.
// Code generated by protoc-gen-gogo. DO NOT EDIT.
// source: main.proto

package test

import (
	fmt "fmt"
	math "math"
	proto "github.com/golang/protobuf/proto"
	_ "google.golang.org/protobuf/types/known/structpb"                <<<<---- structpb first
	_ "google.golang.org/protobuf/types/known/timestamppb"        <<<<---- timestamp second
)

// Reference imports to suppress errors if they are not otherwise used.
var _ = proto.Marshal
var _ = fmt.Errorf
var _ = math.Inf