Hygiene: replace call to rm with clean-webpack-plugin

The invocation of `rm -rf resources/dist` in package.json
(`check-built-assets`) is replaced with clean-webpack-plugin.
The benefit of this change is that calling `npm run build` now works the
same as the `check-built-assets` script.

Bug: T193522
Change-Id: I64f048855ddceb7159279671b2174a7937e169ff

package.json

@@ -9,7 +9,7 @@
     "test": "npm run check-built-assets && npm run test:dev && npm run doc",
     "doc": "jsdoc -c jsdoc.json",
     "doc:start": "nodemon --watch src --exec 'npm run doc'",
-    "check-built-assets": "echo 'CHECKING BUILD SOURCES ARE COMMITTED OR STAGED' && rm -rf resources/dist/ && npm run build && git diff -q resources/dist",
+    "check-built-assets": "echo 'CHECKING BUILD SOURCES ARE COMMITTED OR STAGED' && npm run build && git diff -q resources/dist",
     "coverage": "istanbul cover node_modules/.bin/mw-node-qunit tests/node-qunit/*.js tests/node-qunit/**/*.js",
     "precommit": "npm run test:dev && npm run build && git add resources/dist && bundlesize"
   },
@@ -18,6 +18,7 @@
     "babel-preset-env": "1.6.0",
     "babel-register": "6.24.1",
     "bundlesize": "0.15.3",
+    "clean-webpack-plugin": "0.1.19",
     "eslint-config-wikimedia": "0.5.0",
     "grunt": "1.0.2",
     "grunt-banana-checker": "0.6.0",

webpack.config.js

@@ -1,6 +1,7 @@
 /* global __dirname, process */
 const path = require( 'path' ),
 	webpack = require( 'webpack' ),
+	CleanPlugin = require( 'clean-webpack-plugin' ),
 	PUBLIC_PATH = '/w/extensions/Popups',
 	isProduction = process.env.NODE_ENV === 'production';
 
@@ -62,6 +63,8 @@ const conf = {
 		]
 	},
 	plugins: [
+		new CleanPlugin( 'resources/dist', { verbose: false } ),
+
 		// To generate identifiers that are preserved over builds, webpack supplies
 		// the NamedModulesPlugin (generates comments with file names on bundle)
 		// https://webpack.js.org/guides/caching/#deterministic-hashes