[meta] Make more things pub(crate) instead of pub;

This could help the compiler find unused fields/methods. It didn't find any
during this migration.

cranelift/codegen/meta/src/default_map.rs

@@ -1,7 +1,7 @@
 use std::collections::HashMap;
 use std::hash::Hash;
 
-pub trait MapWithDefault<K, V: Default> {
+pub(crate) trait MapWithDefault<K, V: Default> {
     fn get_or_default(&mut self, k: K) -> &mut V;
 }