Fixes a regression introduced in commit 0adfe95.
* 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.