Hash module IDs to save on bundle size (#6660)

packages/next/build/webpack/plugins/all-modules-identified-plugin.ts

 import { Compiler, Plugin } from 'webpack'
+import { createHash } from 'crypto'
 
 export class AllModulesIdentifiedPlugin implements Plugin {
   apply(compiler: Compiler) {
@@ -8,11 +9,23 @@ export class AllModulesIdentifiedPlugin implements Plugin {
         compilation.hooks.beforeModuleIds.tap(
           'AllModulesIdentifiedPlugin',
           modules => {
-            ;(modules as any[]).forEach(module => {
-              if (module.id != null || !module.identifier) {
+            ;(modules as any[]).forEach(m => {
+              if (m.id != null || !m.identifier) {
                 return
               }
-              module.id = module.identifier()
+
+              const identifier = m
+                .identifier()
+                // Ensure the context isn't included in the hash (this normally
+                // isn't present)
+                .replace(m.context, '')
+
+              // This hashing algorithm is consistent with how the rest of
+              // webpack does it (n.b. HashedModuleIdsPlugin)
+              m.id = createHash('md4')
+                .update(identifier)
+                .digest('hex')
+                .substr(0, 4)
             })
           }
         )