chore!: drop webpack-4 support in v5 (#6598)

1. I don't think I'll ever have the capacity to keep maintaining this
compat plugin. It's better to drop it now than later.
2. The plugin is needed at the time of alpha.0 because many ecosystem
plugins does not support webpack 5. But now I don't think it will be a
major hurdle.
3. The implementation was too hacky.

Author: haoqunjiang

.circleci/config.yml

@@ -49,27 +49,13 @@ jobs:
           at: ~/
       - run: yarn test -p cli,cli-service,cli-shared-utils
 
-  core-webpack-4:
-    <<: *defaults
-    steps:
-      - attach_workspace:
-          at: ~/
-      - run: VUE_CLI_USE_WEBPACK4=true yarn test -p cli,cli-service,cli-shared-utils
-
   typescript:
     <<: *defaults
     steps:
       - attach_workspace:
           at: ~/
       - run: yarn test 'ts(?:\w(?!E2e))+\.spec\.js$'
 
-  typescript-webpack-4:
-    <<: *defaults
-    steps:
-      - attach_workspace:
-          at: ~/
-      - run: VUE_CLI_USE_WEBPACK4=true yarn test 'ts(?:\w(?!E2e))+\.spec\.js$'
-
   plugins:
     <<: *defaults
     steps:
@@ -79,13 +65,6 @@ jobs:
       - run: yarn check-links
       - run: yarn test -p eslint,pwa,babel,babel-preset-app,vuex,router
 
-  plugins-webpack-4:
-    <<: *defaults
-    steps:
-      - attach_workspace:
-          at: ~/
-      - run: VUE_CLI_USE_WEBPACK4=true yarn test -p eslint,pwa,babel,babel-preset-app,vuex,router
-
   tests:
     <<: *defaults
     steps:
@@ -95,14 +74,6 @@ jobs:
       # e2e-nightwatch was left out due to some unknown issues with selenium and the CI image
       - run: yarn test tsPluginE2e
 
-  tests-webpack-4:
-    <<: *defaults
-    steps:
-      - attach_workspace:
-          at: ~/
-      - run: VUE_CLI_USE_WEBPACK4=true yarn test -p unit-mocha,unit-jest,e2e-cypress
-      - run: VUE_CLI_USE_WEBPACK4=true yarn test tsPluginE2e
-
   cli-ui:
     <<: *defaults
     steps:
@@ -116,8 +87,6 @@ jobs:
       - store_artifacts:
           path: /home/circleci/.npm/_logs
 
-  # TODO: cli-ui-webpack-4
-
 workflows:
   version: 2
   test:
@@ -128,34 +97,18 @@ workflows:
           <<: *filters
           requires:
             - install
-      - core-webpack-4:
-          <<: *filters
-          requires:
-            - install
       - typescript:
           <<: *filters
           requires:
             - install
-      - typescript-webpack-4:
-          <<: *filters
-          requires:
-            - install
       - plugins:
           <<: *filters
           requires:
             - install
-      - plugins-webpack-4:
-          <<: *filters
-          requires:
-            - install
       - tests:
           <<: *filters
           requires:
             - install
-      - tests-webpack-4:
-          <<: *filters
-          requires:
-            - install
       - cli-ui:
           <<: *filters
           requires:

docs/core-plugins/webpack-4.md

@@ -60,22 +60,6 @@ Besides the internal changes that are only noticeable for custom configurations,
 1. Named exports from JSON modules are no longer supported. Instead of `import { version } from './package.json'; console.log(version);` use `import package from './package.json'; console.log(package.version);`
 2. Webpack 5 does no longer include polyfills for Node.js modules by default. You shall see an informative error message if your code relies on any of these modules. A detailed list of previously polyfilled modules is also available [here](https://github.com/webpack/webpack/pull/8460/commits/a68426e9255edcce7822480b78416837617ab065).
 
-#### Opt Out to Webpack 4
-
-Considering many ecosystem packages haven't catched up yet, we provided a plugin to opt out to webpack 4 for easier migration.
-
-It's as simple as running
-
-```sh
-vue add webpack-4
-```
-
-at the project root.
-
-Underlyingly, it uses the [`resolutions`](https://classic.yarnpkg.com/en/docs/selective-version-resolutions) field for Yarn and PNPM users, and [`module-alias`](https://github.com/ilearnio/module-alias) for NPM users.
-
-Though both work in all our tests, please be aware that the `module-alias` approach is still considered hacky, and may not be as stable as the `"resolutions"` one.
-
 #### Changes to the `build` command and modern mode
 
 Starting with v5.0.0-beta.0, running `vue-cli-service build` will automatically generate different bundles based on your browserslist configurations.

docs/migrations/migrate-from-v4.md

@@ -7,10 +7,3 @@ module.exports = {
     '**/__tests__/**/*.spec.js'
   ]
 }
-
-if (process.env.VUE_CLI_USE_WEBPACK4) {
-  module.exports.moduleNameMapper = {
-    '^webpack$': 'webpack-4',
-    '^webpack/(.*)': 'webpack-4/$1'
-  }
-}