Fix check and show answer for split

LMS-11237

@cpennington I had to impl resources_fs in split b/c capa_problem accesses it. imnsho, we should review all filesystem stuff. Anyway, my concern was using the course as the subdir could cause collisions w/ other == named courses, but that should be a current problem too; so, it's probably ok?

I'm still not clear what this directory is used for. I appears to be used for storing student submissions in which case it seems like name collision is an existing problem.