util: remove dependencies on main crate

Only items from `fish_widestring`'s prelude are used, so depend on that
directly, to prepare for extraction.

Part of #12316

src/util.rs

@@ -1,6 +1,6 @@
 //! Generic utilities library.
 
-use crate::prelude::*;
+use fish_widestring::prelude::*;
 use rand::{SeedableRng, rngs::SmallRng};
 use std::cmp::Ordering;
 use std::time;
@@ -260,7 +260,7 @@ pub fn find_subslice<T: PartialEq>(
 #[cfg(test)]
 mod tests {
     use super::wcsfilecmp;
-    use crate::prelude::*;
+    use fish_widestring::prelude::*;
     use std::cmp::Ordering;
 
     /// Verify the behavior of the `wcsfilecmp()` function.