Use relative import

[LecrisUT]

I just realized the cause of the failure of https://github.com/teemtee/tmt/actions/runs/9726385087/job/26844764885?pr=3062

Basically because the project does not use src-layout and absolute import paths are used, it is very easy to be out-of-sync, e.g. in that PR the dependencies and entry_point come from the pre-commit, but the source for tmt module comes from the current directory instead. #3117 intends to fix part of that issue, but if other users have a tmt module in their top-level, than things will get wacky again.