Fixes a regression introduced in
7208995426 whereby (gnu system
file-systems) would pull in (gnu packages …) module, which in turn
breaks when importing things like (gnu build shepherd).
* gnu/system/file-systems.scm (file-system-type-predicate): Export.
(file-system-packages): Move to...
* gnu/system/linux-initrd.scm (file-system-packages): ... here. Add
docstring.
* gnu/services/base.scm: Use it.
* tests/file-systems.scm ("does not pull (gnu packages …)"): New test.