While working on MDL-75111, about to properly name a number of unit test files and put them under their correct namespace, a few annoyances were found. Here it's the list (copied from that issue):

B) Some annoyances - surely deserve a MDL-xxxx to be created

• All the classes and tests under lib/filebrowser and lib/filestorage
  belong to core, not to core_files. Some day we should move
  them to their correct subsystem.
• All the classes and tests under lib/grade belong to core, not
  to core_grades. Some day we should move them to their correct
  subsystem.
• The core_grades_external class and its \core\grades_external_test
  unit test should belong to the grades subsystem or, alternatively,
  to \core\external, they both should be moved together.
• The core_grading_external class and its \core\grading_external_test
  unit test should belong to the grading subsystem or, alternatively,
  to \core\external, they both should be moved together.
• The \core\message\message and \core\message\inbound (may be others)
  classes, and their associated tests should go to the core_message
  subsystem.
• The core_user class, and its associated tests should go to the
  core_user subsystem.
• The \core\update namespace is plain wrong (update is not valid API)
  and needs action 1) create it or 2) move elsewhere.

This issue is about to consider if we should perform the moves/fixes, how to do that in a BC way and, surely try to detect more cases and proceed.

Maybe it wouldn't be crazy to convert this to an Epic and then address the problems case by case. In any case, master-only.

Ciao