Issue1147 Use pathlib

[FlorianPommerening]

No description provided.

[FlorianPommerening]

Unrelated to the issue but this was causing a failing style check locally.

[speckdavid]

I checked the new logic for storing arguments for validation in args.validate_inputs. It looks good to me. The only somewhat strange behavior I noticed is that one can call translate and validate without search (e.g., ./fast-downward.py --translate --validate problem.pddl). This validates an old plan file sas_plan (possibly from another planning task) if it exists, or none if it doesn't. This behavior existed before in the main branch and isn't directly related to the pull request. Just asking if we want this behavior or if we should fix it.

[FlorianPommerening]

The only somewhat strange behavior I noticed is that one can call translate and validate without search [...] This behavior existed before in the main branch and isn't directly related to the pull request.

We discussed this offline and decided to move this to a separate issue.