fix(v2): fix Webpack persistent caching (evict on swizzle/alias/config change) (#5047)

* webpack upgrade

* refactor docusaurus-utils hash fns

* Fix webpack cache eviction problems on config/aliases/swizzle changes

* Move/Rename InitPlugin type

* fix TS typos

* Add tests for webpack aliases

* fix windows tests

packages/docusaurus-utils/src/hashUtils.ts

@@ -0,0 +1,37 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {createHash} from 'crypto';
+import {kebabCase} from 'lodash';
+import {shortName, isNameTooLong} from './pathUtils';
+
+export function md5Hash(str: string): string {
+  return createHash('md5').update(str).digest('hex');
+}
+
+export function simpleHash(str: string, length: number): string {
+  return md5Hash(str).substr(0, length);
+}
+
+// Based on https://github.com/gatsbyjs/gatsby/pull/21518/files
+/**
+ * Given an input string, convert to kebab-case and append a hash.
+ * Avoid str collision.
+ * Also removes part of the string if its larger than the allowed
+ * filename per OS. Avoids ERRNAMETOOLONG error.
+ */
+export function docuHash(str: string): string {
+  if (str === '/') {
+    return 'index';
+  }
+  const shortHash = simpleHash(str, 3);
+  const parsedPath = `${kebabCase(str)}-${shortHash}`;
+  if (isNameTooLong(parsedPath)) {
+    return `${shortName(kebabCase(str))}-${shortHash}`;
+  }
+  return parsedPath;
+}