Remove calls to system_page_attachments()

A few things going on here:

- the system/base library shouldn't be in system module at all, moved to core.libraries.yml and core/misc.

- we probably need to do the same for system.admin and system.maintenance since these are called by template_preprocess_maintenance_page(), but that might not be necessary to fix this specific issue and dependencies, potentially can happen in a follow-up.

Tests are green. I implemented bc for anything declaring system/base as a library dependency but not for library overrides - personally I think a change record should be enough for those given it's essentially an alter hook.

- the system/base library shouldn't be in system module at all, moved to core.libraries.yml and core/misc.

I bet system/base precedes the "core extension" existing! 👴😄

Ah, you specifically commented on that in #5. Well … if a release manager thinks that's okay, then I'm fine with it too — you're totally right that it's far less common to do library overrides. And the changes in claro.info.yml can serve as an inspiration for how to do this.

Just one remaining question regarding BC: how can a theme be compatible with both the "before" and "after" states in a single version?

Looking at system/base I found a tonne of issues which are now documented in 📌 Move system/base component CSS to respective libraries where they exist Fixed and related issues. If we do those issues, there will be much less change to do here, so I think we should soft-postpone this issue to avoid moving things two or three times.

Postponing this on 📌 Move system/base component CSS to respective libraries where they exist Fixed 📌 Refactor system/base library Needs work , we might even be able to just drop the calls altogether if we do those without having to manually load the libraries in the installer any more, or at least, it would be a very short list.