Draft: [RFC] main/alpine-base: don't hard-depend on openrc

This is a first attempt to try to loose the hard-dependency of alpine-base on openrc. The reason I've attempted this is because we are experimenting downstream with systemd, but we still like the alpine-base package. I wondered for a while whether I should be pushing this here, or instead just workaround it downstream (e.g: taking most alpine-base dependencies). I finally decided to send this MR because I believe there might be uses in alpine for this abstraction:

• Probably reduces the amount of hacks other people need to properly boot with other init systems.
• If alpine ever moves out of openrc to a modern init system, there is probably some testing and work to be done, and this abstraction will ease that process

I understand if this is rejected, even though it might be a shame for us.

Also, I have not completely figured out what to do with the busybox-mdev-openrc dependency. It feels like after #14877 (closed) the virtual dev-openrc is of very little use, as it's not referenced anywhere. I kept it here because those files won't hurt, but I'm still trying to figure out what would be the best strategy (like maybe using a non-virtual provider).