refactor: follow dart layout conventions

@@ -5,14 +5,5 @@
 
 library flutter_screenutil;
 
-import 'dart:math';
-import 'dart:ui' as ui;
-import 'dart:ui';
-
-import 'package:flutter/material.dart';
-
-part 'screen_util.dart';
-
-part 'screenutil_init.dart';
-
-part 'size_extension.dart';
+export 'src/screen_util.dart';
+export 'src/screenutil_init.dart';

@@ -2,7 +2,10 @@
  * Created by 李卓原 on 2018/9/29.
  * email: zhuoyuan93@gmail.com
  */
-part of flutter_screenutil;
+
+import 'dart:math';
+
+import 'package:flutter/material.dart';
 
 class ScreenUtil {
   static const Size defaultSize = Size(360, 690);

-part of flutter_screenutil;
+import 'package:flutter/cupertino.dart';
+
+import 'screen_util.dart';
 
 class ScreenUtilInit extends StatelessWidget {
   /// A helper widget that initializes [ScreenUtil]
-  ScreenUtilInit({
+  const ScreenUtilInit({
     required this.builder,
     this.designSize = ScreenUtil.defaultSize,
     this.splitScreenMode = false,

-part of flutter_screenutil;
+import 'dart:math';
+
+import 'package:flutter/material.dart';
+
+import '../flutter_screenutil.dart';
 
 extension SizeExtension on num {
   ///[ScreenUtil.setWidth]