Make a module `pub(crate)` rather than `pub`

zebra-state/src/service/check.rs

@@ -13,9 +13,10 @@ use crate::{PreparedBlock, ValidateContextError};
 
 use super::check;
 
-pub mod difficulty;
 use difficulty::{AdjustedDifficulty, POW_MEDIAN_BLOCK_SPAN};
 
+pub(crate) mod difficulty;
+
 /// Check that `block` is contextually valid for `network`, based on the
 /// `finalized_tip_height` and `relevant_chain`.
 ///