refactor(@angular-devkit/build-angular): use library output for stylesheet child compilations

This change allows the result of the stylesheet Webpack compilation to be explicitly available for access after the output source code is generated.  The previous method relied on assumptions within the code generated by Webpack and no longer worked with Webpack 5.

Author: clydin

packages/ngtools/webpack/src/resource_loader.ts

@@ -15,7 +15,7 @@ import { forwardSlashPath } from './utils';
 
 const NodeTemplatePlugin = require('webpack/lib/node/NodeTemplatePlugin');
 const NodeTargetPlugin = require('webpack/lib/node/NodeTargetPlugin');
-const LoaderTargetPlugin = require('webpack/lib/LoaderTargetPlugin');
+const LibraryTemplatePlugin = require('webpack/lib/LibraryTemplatePlugin');
 const SingleEntryPlugin = require('webpack/lib/SingleEntryPlugin');
 
 
@@ -68,7 +68,7 @@ export class WebpackResourceLoader {
     new NodeTemplatePlugin(outputOptions).apply(childCompiler);
     new NodeTargetPlugin().apply(childCompiler);
     new SingleEntryPlugin(this._context, filePath).apply(childCompiler);
-    new LoaderTargetPlugin('node').apply(childCompiler);
+    new LibraryTemplatePlugin('resource', 'var').apply(childCompiler);
 
     childCompiler.hooks.thisCompilation.tap('ngtools-webpack', (compilation: any) => {
       compilation.hooks.additionalAssets.tapAsync('ngtools-webpack',
@@ -155,13 +155,13 @@ export class WebpackResourceLoader {
 
   private async _evaluate({ outputName, source }: CompilationOutput): Promise<string> {
       // Evaluate code
-      const evaluatedSource = vm.runInNewContext(source, undefined, { filename: outputName });
-      if (typeof evaluatedSource === 'object' && typeof evaluatedSource.default === 'string') {
-        return evaluatedSource.default;
-      }
+      const context: { resource?: string | { default?: string } } = {};
+      vm.runInNewContext(source, context, { filename: outputName });
 
-      if (typeof evaluatedSource === 'string') {
-        return evaluatedSource;
+      if (typeof context.resource === 'string') {
+        return context.resource;
+      } else if (typeof context.resource?.default === 'string') {
+        return context.resource.default;
       }
 
       throw new Error(`The loader "${outputName}" didn't return a string.`);