feat: [BREAKING_CHANGE] remove compatible support loadExtend (#12)

lib/loader/mixin/extend.js

@@ -1,6 +1,5 @@
 'use strict';
 
-const fs = require('fs');
 const path = require('path');
 const interopRequire = require('interop-require');
 const debug = require('debug')('egg-core:extend');
@@ -73,14 +72,7 @@ module.exports = {
    */
   loadExtend(name, proto) {
     // 获取需要加载的文件
-    const filepaths = this.getLoadUnits()
-      .map(unit => {
-        let pluginExtendsPath = path.join(unit.path, 'app/extend');
-        if (!fs.existsSync(pluginExtendsPath)) {
-          pluginExtendsPath = path.join(unit.path, 'app');
-        }
-        return path.join(pluginExtendsPath, name);
-      });
+    const filepaths = this.getLoadUnits().map(unit => path.join(unit.path, 'app/extend', name));
     const mergeRecord = new Map();
     for (const filepath of filepaths) {
       if (!utils.existsModule(filepath)) {