Where has module-specified HOOK_preprocess_page gone in D8? I'm making a quick module whilst playing with D8, which I want to preprocess Stylus into CSS. The obvious choice for me, coming from D7, would be to use hook_preprocess_page, but it seems to have been removed from D8, or at least only in themes and not modules.

So just to reiterate, I have my module stylus, my file stylus.module and the function stylus_preprocess_page(&amp;$variables), but no dice.

What would be the best D8 alternative?
This hook has not been removed, there are still preprocess hooks for modules:

https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Render!theme.api.php/function/hook_preprocess_HOOK/8.2.x