chore!: ensure consistent directory structure for all modes (#4323)

BREAKING CHANGE:
Changing directory strcuture, though now becoming more intuitive, may
still break users' workflows, so it's considered a breaking change.

Fixes #4215.
Fixes #3767.
Fixes #4234

Author: haoqunjiang

packages/@vue/cli-service/lib/config/app.js

@@ -23,6 +23,16 @@ module.exports = (api, options) => {
     const isLegacyBundle = process.env.VUE_CLI_MODERN_MODE && !process.env.VUE_CLI_MODERN_BUILD
     const outputDir = api.resolve(options.outputDir)
 
+    const getAssetPath = require('../util/getAssetPath')
+    const filename = getAssetPath(
+      options,
+      `js/[name]${isLegacyBundle ? `-legacy` : ``}${isProd && options.filenameHashing ? '.[contenthash:8]' : ''}.js`
+    )
+    webpackConfig
+      .output
+        .filename(filename)
+        .chunkFilename(filename)
+
     // code splitting
     if (process.env.NODE_ENV !== 'test') {
       webpackConfig

packages/@vue/cli-service/lib/config/prod.js

@@ -1,19 +1,9 @@
 module.exports = (api, options) => {
   api.chainWebpack(webpackConfig => {
     if (process.env.NODE_ENV === 'production') {
-      const isLegacyBundle = process.env.VUE_CLI_MODERN_MODE && !process.env.VUE_CLI_MODERN_BUILD
-      const getAssetPath = require('../util/getAssetPath')
-      const filename = getAssetPath(
-        options,
-        `js/[name]${isLegacyBundle ? `-legacy` : ``}${options.filenameHashing ? '.[contenthash:8]' : ''}.js`
-      )
-
       webpackConfig
         .mode('production')
         .devtool(options.productionSourceMap ? 'source-map' : false)
-        .output
-          .filename(filename)
-          .chunkFilename(filename)
 
       // keep module.id stable when vendor modules does not change
       webpackConfig