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

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

Author: AndrewKushnir

packages/platform-server/test/integration_spec.ts

@@ -61,7 +61,7 @@ import {
   provideServerRendering,
   renderModule,
   ServerModule,
-} from '@angular/platform-server';
+} from '../index';
 import {provideRouter, RouterOutlet, Routes} from '@angular/router';
 import {Observable} from 'rxjs';
 

packages/platform-server/test/transfer_state_spec.ts

@@ -8,7 +8,7 @@
 
 import {Component, makeStateKey, NgModule, TransferState} from '@angular/core';
 import {BrowserModule} from '@angular/platform-browser';
-import {renderModule, ServerModule} from '@angular/platform-server';
+import {renderModule, ServerModule} from '../index';
 
 describe('transfer_state', () => {
   const defaultExpectedOutput =

packages/platform-server/testing/src/server.ts

@@ -12,7 +12,7 @@ import {BrowserDynamicTestingModule} from '@angular/platform-browser-dynamic/tes
 import {
   ɵINTERNAL_SERVER_PLATFORM_PROVIDERS as INTERNAL_SERVER_PLATFORM_PROVIDERS,
   ɵSERVER_RENDER_PROVIDERS as SERVER_RENDER_PROVIDERS,
-} from '@angular/platform-server';
+} from '../../index';
 
 const INTERNAL_SERVER_DYNAMIC_PLATFORM_TESTING_PROVIDERS: StaticProvider[] = [
   ...INTERNAL_BROWSER_DYNAMIC_PLATFORM_PROVIDERS,