fix(@angular-devkit/build-angular): generate sourcemaps with relative paths in monorepo

With this change we fix the issue that when having a monorepo style workspace, sourcemaps sources references absolute paths.

Closes #17046

Author: alan-agius4

packages/angular_devkit/build_angular/src/angular-cli-files/models/webpack-configs/common.ts

@@ -489,7 +489,7 @@ export function getCommonConfig(wco: WebpackConfigOptions): Configuration {
       ],
       plugins: [PnpWebpackPlugin.moduleLoader(module)],
     },
-    context: projectRoot,
+    context: root,
     entry: entryPoints,
     output: {
       futureEmitAssets: true,

tests/legacy-cli/e2e/tests/build/relative-sourcemap.ts

@@ -0,0 +1,20 @@
+import * as fs from 'fs';
+
+import { isAbsolute } from 'path';
+import { ng } from '../../utils/process';
+
+export default async function () {
+  // General secondary application project
+  await ng('generate', 'application', 'secondary-project', '--skip-install');
+  await ng('build', 'secondary-project');
+
+
+  await ng('build', '--output-hashing=none', '--source-map');
+  const content = fs.readFileSync('./dist/secondary-project/main.js.map', 'utf8');
+  const {sources} = JSON.parse(content);
+  for (const source of sources) {
+      if (isAbsolute(source)) {
+        throw new Error(`Expected ${source} to be relative.`);
+      }
+  }
+}