fix(material/schematics): remove node_modules prefix from prebuilt theme

The Angular CLI supports resolving node modules without the need of specifying `node_modules` directory. This also needed to add support for Yarn PNP which doesn't create a `node_modules` directory.

Closes #25242

Author: alan-agius4

src/material/schematics/ng-add/index.spec.ts

@@ -393,8 +393,7 @@ describe('ng-add schematic', () => {
 
   describe('theme files', () => {
     /** Path to the default prebuilt theme file that will be added when running ng-add. */
-    const defaultPrebuiltThemePath =
-      './node_modules/@angular/material/prebuilt-themes/indigo-pink.css';
+    const defaultPrebuiltThemePath = '@angular/material/prebuilt-themes/indigo-pink.css';
 
     /** Writes a specific style file to the workspace in the given tree */
     function writeStyleFileToWorkspace(tree: Tree, stylePath: string) {

src/material/schematics/ng-add/theming/theming.ts

@@ -109,8 +109,7 @@ async function insertCustomTheme(
 
 /** Insert a pre-built theme into the angular.json file. */
 function insertPrebuiltTheme(project: string, theme: string, logger: logging.LoggerApi): Rule {
-  // Path needs to be always relative to the `package.json` or workspace root.
-  const themePath = `./node_modules/@angular/material/prebuilt-themes/${theme}.css`;
+  const themePath = `@angular/material/prebuilt-themes/${theme}.css`;
 
   return chain([
     addThemeStyleToTarget(project, 'build', themePath, logger),