perf(@angular-devkit/build-angular): only import esbuild watcher when in watch mode

When using the esbuild-based browser application builder, all the watch-related code
is now only imported when the watch mode is enabled. This removes the need for Node.js
to resolve and load code that will not be used.

(cherry picked from commit 8daf3a5)

Author: clydin

packages/angular_devkit/build_angular/src/builders/browser-esbuild/index.ts

@@ -32,7 +32,7 @@ import { NormalizedBrowserOptions, normalizeOptions } from './options';
 import { shutdownSassWorkerPool } from './sass-plugin';
 import { Schema as BrowserBuilderOptions } from './schema';
 import { createStylesheetBundleOptions } from './stylesheets';
-import { ChangedFiles, createWatcher } from './watcher';
+import type { ChangedFiles } from './watcher';
 
 interface RebuildState {
   codeRebuild?: BundlerContext;
@@ -673,6 +673,7 @@ export async function* buildEsbuildBrowser(
   }
 
   // Setup a watcher
+  const { createWatcher } = await import('./watcher');
   const watcher = createWatcher({
     polling: typeof userOptions.poll === 'number',
     interval: userOptions.poll,