file-systems: Refactor file-system predicates.
* gnu/build/file-systems.scm (partition-field-reader,
read-partition-field, %partition-label-readers,
%partition-uuid-readers, read-partition-label, read-partition-uuid):
New variables.
(partition-predicate, partition-label-predicate,
partition-uuid-predicate, luks-partition-uuid-predicate): Use
partition field readers.
(find-partition): New variable.
(find-partition-by-label, find-partition-by-uuid,
find-partition-by-luks-uuid): Use find-partition-by.
ab4e939c50