Deprecate loaders (#5294)

* deprecate null-loader for webpack 5

* deprecate mocha-loader

* deprecate istanbul-instrumenter-loader

* clean up

Author: chenxsan

src/content/loaders/index.mdx

@@ -10,6 +10,7 @@ contributors:
   - snitin315
   - anshumanv
   - jamesgeorge007
+  - chenxsan
 ---
 
 webpack enables use of [loaders](/concepts/loaders) to preprocess files. This allows you to bundle any static resource way beyond JavaScript. You can easily write your own loaders using Node.js.
@@ -56,10 +57,6 @@ Loaders are activated by using `loadername!` prefixes in `require()` statements,
 - [`postcss-loader`](/loaders/postcss-loader) Loads and transforms a CSS/SSS file using [PostCSS](http://postcss.org)
 - [`stylus-loader`](/loaders/stylus-loader/) Loads and compiles a Stylus file
 
-## Testing
-
-- [`mocha-loader`](/loaders/mocha-loader) Tests with [mocha](https://mochajs.org/) (Browser/NodeJS)
-
 ## Frameworks
 
 - [`vue-loader`](https://github.com/vuejs/vue-loader) Loads and compiles [Vue Components](https://vuejs.org/v2/guide/components.html)

src/utilities/constants.js

@@ -20,6 +20,9 @@ const excludedLoaders = [
   'webpack-contrib/raw-loader',
   'webpack-contrib/url-loader',
   'webpack-contrib/file-loader',
+  'webpack-contrib/null-loader',
+  'webpack-contrib/mocha-loader',
+  'webpack-contrib/istanbul-instrumenter-loader',
 ];
 const excludedPlugins = [
   'webpack-contrib/component-webpack-plugin',

webpack.ssg.js

@@ -94,6 +94,12 @@ module.exports = (env) =>
           'loaders/url-loader': 'https://v4.webpack.js.org/loaders/url-loader',
           'loaders/file-loader':
             'https://v4.webpack.js.org/loaders/file-loader',
+          'loaders/null-loader':
+            'https://v4.webpack.js.org/loaders/null-loader/',
+          'loaders/mocha-loader':
+            'https://v4.webpack.js.org/loaders/mocha-loader/',
+          'loaders/istanbul-instrumenter-loader':
+            'https://v4.webpack.js.org/loaders/istanbul-instrumenter-loader/',
         },
       }),
       new CopyWebpackPlugin({