`ng_module` will now include an `src/r3_symbol.d.ts` when compiling the core package under `ngc` togather with `dts bundling`, This is due that `ngcc` relies on this file to be present, but the `r3_symbols` file which is not part of our public api.
With this change, we can now ship an addition dts file which is flattened.
PR Close#28884