fix(@ngtools/webpack): only track actual resource file dependencies

Webpack's `fileDependencies` Set could contain directories as well as files. The directories were previously stored and incorrectly used during cache invalidation which resulted in excessive cache validation.
This change attempts to skip directories by ignoring any `fileDependencies` entry that does not have a file extension.

(cherry picked from commit b8fc1dc)

packages/ngtools/webpack/src/resource_loader.ts

@@ -6,6 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 import { createHash } from 'crypto';
+import * as path from 'path';
 import * as vm from 'vm';
 import { Compilation, EntryPlugin, NormalModule, library, node, sources } from 'webpack';
 import { normalizePath } from './ivy/paths';
@@ -206,6 +207,14 @@ export class WebpackResourceLoader {
           this._fileDependencies.set(filePath, new Set(childCompilation.fileDependencies));
           for (const file of childCompilation.fileDependencies) {
             const resolvedFile = normalizePath(file);
+
+            // Skip paths that do not appear to be files (have no extension).
+            // `fileDependencies` can contain directories and not just files which can
+            // cause incorrect cache invalidation on rebuilds.
+            if (!path.extname(resolvedFile)) {
+              continue;
+            }
+
             const entry = this._reverseDependencies.get(resolvedFile);
             if (entry) {
               entry.add(filePath);