services: <shepherd-service> no longer has an 'imported-modules' field.

services: <shepherd-service> no longer has an 'imported-modules' field.

* gnu/services/shepherd.scm (<shepherd-service>)[imported-modules]:
Remove.
(%default-imported-modules): Make private.
(shepherd-service-file): Use 'with-imported-modules'.
(shepherd-configuration-file): Remove 'modules' and the calls to
'imported-modules' and 'compiled-modules'.  Use
'with-imported-modules' instead.
* doc/guix.texi (Shepherd Services): Adjust accordingly.
* gnu/services/base.scm (file-system-shepherd-service): Use
'with-imported-modules'.  Remove 'imported-modules' field.
* gnu/system/mapped-devices.scm (device-mapping-service-type): Remove
'imported-modules'.
(open-luks-device): Use 'with-imported-modules'.
* gnu/tests.scm (marionette-shepherd-service): Remove 'imported-modules'
field and use 'with-imported-modules'.