fix(ivy): ngtsc fails building flat module out on windows (#27993)

`ngtsc` currently fails building a flat module out file on Windows because it generates an invalid flat module TypeScript source file. e.g:

```ts
5 export * from './C:\Users\Paul\Desktop\test\src\export';
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

This is because `path.posix.relative` does not properly with non-posix paths, and only expects posix paths in order to work.

PR Close #27993

packages/compiler-cli/test/ngtsc/ngtsc_spec.ts

@@ -1610,6 +1610,17 @@ describe('ngtsc behavioral tests', () => {
       expect(dtsContents).toContain('/// <amd-module name="@mymodule" />');
     });
 
+    it('should generate a proper flat module index file when nested', () => {
+      env.tsconfig({
+        'flatModuleOutFile': './public-api/index.js',
+      });
+
+      env.write('test.ts', `export const SOME_EXPORT = 'some-export'`);
+      env.driveMain();
+
+      expect(env.getContents('./public-api/index.js')).toContain(`export * from '../test';`);
+    });
+
     it('should report an error when a flat module index is requested but no entrypoint can be determined',
        () => {
          env.tsconfig({'flatModuleOutFile': 'flat.js'});