Use Run::Parts instead of calling run-parts directly

Add according (build-) dependencies.
Refactoring: Replace run_parts_files by Run::Parts->concat

Merge remaining necessary parts of run_parts_files into
run_parts_lines and let Run::Parts->concat do most of run_parts_lines'
work.

Remove t/020_runparts.t (which solely tested run_parts_files).
Remove (build-) dependencies on libfile-slurp-perl

They're no more needed.