[problem] Rename Problem.name to Problem.id

[mpsijm]

The Problem.name field left me confused just a bit too often, as I thought it would contain the human-readable name of the problem instead of the problem's ID/"short name" 😇

[thorehusfeldt]

Related recent discussion at Kattis/problem-package-format#99 (comment) . There is more-or-less up-to-date terminology on “my” fork: https://problem-package-format-copy.readthedocs.io/en/latest/format.html#directory-structure .

I think the consensus it refer to this concept (which also give the root-level directory name) the (problem) package name. So an alternative would be Problem.package_name .