refactor(shutdown): move module specific code out of base module

[LaszloGombos]

dracut-lib.sh and the base module now does not have a knowledge of oldroot and plymouth.

It improves the separation of concerns between the base module and the shutdown module.

Checklist

• [X] I have tested it locally

[johannbg]

Let's get @zboszor take on this as well.

[stale[bot]]

This issue is being marked as stale because it has not had any recent activity. It will be closed if no further activity occurs. If this is still an issue in the latest release of Dracut and you would like to keep it open please comment on this issue within the next 7 days. Thank you for your contributions.

[LaszloGombos]

not stale