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/src/ngtsc/entry_point/test/entry_point_spec.ts

@@ -0,0 +1,28 @@
+
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {findFlatIndexEntryPoint} from '../src/logic';
+
+describe('entry_point logic', () => {
+
+  describe('findFlatIndexEntryPoint', () => {
+
+    it('should use the only source file if only a single one is specified',
+       () => { expect(findFlatIndexEntryPoint(['/src/index.ts'])).toBe('/src/index.ts'); });
+
+    it('should use the shortest source file ending with "index.ts" for multiple files', () => {
+      expect(findFlatIndexEntryPoint([
+        '/src/deep/index.ts', '/src/index.ts', '/index.ts'
+      ])).toBe('/index.ts');
+    });
+
+    it('should normalize the path separators for the found entry point',
+       () => { expect(findFlatIndexEntryPoint(['\\src\\index.ts'])).toBe('/src/index.ts'); });
+  });
+});