[draft as this needs some cleanups and tests first but the idea is there]

This commit adds support to build ISO images for the bootc distro.

The core idea is that the `distro.Distro` abstraction can have custom depsolvers. This makes sense anyway if we ever want to support non-rpm distros. For bootc we need it because we run the dnf from inside the container to be sure that we get all custom plugins/repo config etc.

This is otherwise a (mostly) 1-1 copy of the code we have already in bib.

/jira-epic HMS-8839