system: File systems depend on their corresponding device mappings.

Fixes a regression introduced in commit 0adfe95a.

* gnu/system.scm (other-file-system-services)[requirements]: Remove.
  [add-dependencies]: New procedure.
  Use it.
* gnu/system/file-systems.scm (<file-system>)[dependencies]: Update
  comment.
* gnu/services/base.scm (mapped-device->dmd-service-name,
  dependency->dmd-service-name): New procedures.
  (file-system-service-type): Use it.