This enabled dts flattening in the final distrubutable package.
Notes:
- For the time being this is an opt-in feature via the `ng_module` attribute `bundle_dts`, however in the near future this will be turned on by default.
- This only supports the legacy compiler `ngc`, as `ngtsc` emits namespaced imports `import * as __` from local modules which is not supported for the time being by API Extractor. See: https://github.com/Microsoft/web-build-tools/issues/1029
Ref: TOOL-611
PR Close#28588
