A further clarification about this-- when CMS is running, xml_module_store.data_dir and draft_dir end up being absolute paths. ImportSystem concatenates them together, and the concatenation is smart enough to notice that one path is contained in the other (so the concatenation is a no-op). However, when the tests run, draft_dir is a relative path (including part of data_dir), and the concatenation ends up wrong-- like common/test/data/common/test/data/test_course/drafts. ImportSystem works fine for non-draft module store, both in test and when running CMS, because course_dir does not contain part of data_dir (it is just test_course).

Therefore I made this change to make the behavior of the draft_course_dir match the behavior of the course_dir for non-draft modulestore. This allows it to work for both tests and when running CMS.