This is a first step towards moving bootc images into the "images" library.

Eventually we want to move bootc image types into YAML too (like https://github.com/osbuild/images/pull/1656) is doing. But with this first step its a lot easier to move.

90% of this is moving code from bib (with tiny adjustments) into images.

Thanks to Achilleas and Simon for suggesting this simplified strategy.

The functional/end-to-end tests are running in https://github.com/osbuild/bootc-image-builder/pull/1014 as part of the bib test-suite (all end-to-end tests work there)

/jira-epic HMS-8839