refactor(@angular-devkit/build-webpack): cleanup type usage

This change removes some unneeded casts and provides support for building against Webpack 5.  Webpack 5 typings currently do not contain certain types (for example, `Compiler.Handler`).  Webpack 5 also uses `Set` in many places instead of arrays.

Author: clydin

packages/angular_devkit/build_webpack/src/utils.ts

@@ -22,8 +22,8 @@ export function getEmittedFiles(compilation: webpack.compilation.Compilation): E
   const files: EmittedFiles[] = [];
 
   // adds all chunks to the list of emitted files such as lazy loaded modules
-  for (const chunk of compilation.chunks as webpack.compilation.Chunk[]) {
-    for (const file of chunk.files as string[]) {
+  for (const chunk of compilation.chunks as Iterable<webpack.compilation.Chunk>) {
+    for (const file of chunk.files) {
       files.push({
         // The id is guaranteed to exist at this point in the compilation process
         // tslint:disable-next-line: no-non-null-assertion

packages/angular_devkit/build_webpack/src/webpack/index.ts

@@ -52,7 +52,7 @@ export function runWebpack(
 
   return createWebpack(config).pipe(
     switchMap(webpackCompiler => new Observable<BuildResult>(obs => {
-      const callback: webpack.Compiler.Handler = (err, stats) => {
+      const callback = (err: Error | undefined, stats: webpack.Stats) => {
         if (err) {
           return obs.error(err);
         }