refactor(platform-browser-dynamic): switching to relative imports within the platform-browser-dynamic package

This commit updates scripts within `packages/platform-browser-dynamic` to relative imports as a prep work to the upcoming infra updates.

Author: AndrewKushnir

packages/platform-browser-dynamic/test/resource_loader/resource_loader_impl_spec.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.dev/license
  */
 
-import {ResourceLoaderImpl} from '@angular/platform-browser-dynamic/src/resource_loader/resource_loader_impl';
+import {ResourceLoaderImpl} from '../../src/resource_loader/resource_loader_impl';
 
 if (isBrowser) {
   describe('ResourceLoaderImpl', () => {

packages/platform-browser-dynamic/test/testing_public_browser_spec.ts

@@ -9,7 +9,7 @@
 import {ResourceLoader} from '@angular/compiler';
 import {Compiler, Component, NgModule} from '@angular/core';
 import {fakeAsync, inject, TestBed, tick, waitForAsync} from '@angular/core/testing';
-import {ResourceLoaderImpl} from '@angular/platform-browser-dynamic/src/resource_loader/resource_loader_impl';
+import {ResourceLoaderImpl} from '../src/resource_loader/resource_loader_impl';
 
 // Components for the tests.
 class FancyService {

packages/platform-browser-dynamic/testing/src/testing.ts

@@ -7,7 +7,7 @@
  */
 
 import {createPlatformFactory, NgModule, PlatformRef, StaticProvider} from '@angular/core';
-import {platformBrowserDynamic} from '@angular/platform-browser-dynamic';
+import {platformBrowserDynamic} from '../../index';
 import {BrowserTestingModule} from '@angular/platform-browser/testing';
 
 /**