`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
070fca1591