fix(app-plus): add usingComponents(question/87100,87294,87100)

packages/webpack-uni-mp-loader/lib/plugin/generate-json.js

@@ -91,6 +91,9 @@ module.exports = function generateJson (compilation) {
   const jsonFileMap = getChangedJsonFileMap()
   for (let name of jsonFileMap.keys()) {
     const jsonObj = JSON.parse(jsonFileMap.get(name))
+    if (process.env.UNI_PLATFORM === 'app-plus') { // App平台默认增加usingComponents,激活__wxAppCode__
+      jsonObj.usingComponents = {}
+    }
     // customUsingComponents
     if (jsonObj.customUsingComponents && Object.keys(jsonObj.customUsingComponents).length) {
       jsonObj.usingComponents = Object.assign(jsonObj.customUsingComponents, jsonObj.usingComponents)