refactor(scp): factorize common codes

[akinomyoga]

This is another PR separated from #1357. This commit is just trivial refactoring, but the next commit is nontrivial. I decided to submit this single commit before submitting the rest.

[akinomyoga]

I realized that fe7a3f9 is a related commit, which was added after @yedayak's comment at #1357 (comment), so I pushed the commit. If a fixed positional argument is preferred, the commit can be dropped. Otherwise, this extra commit can finally be squashed at the merging time.

[scop]

I like the -d-as-option approach, let's keep it. A couple of suggestions for another followup commit, pre-approved with or without it added.

[akinomyoga]

I applied the door changes. I also squashed the second commit (the change for the -d command-line flag) into the first commit, while rebasing the PR on top of the latest main branch.

[akinomyoga]

Thanks!