[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/cdsl/cpu_modes.rs

@@ -5,7 +5,7 @@ use crate::cdsl::encodings::Encoding;
 use crate::cdsl::types::{LaneType, ValueType};
 use crate::cdsl::xform::{TransformGroup, TransformGroupIndex};
 
-pub struct CpuMode {
+pub(crate) struct CpuMode {
     pub name: &'static str,
     default_legalize: Option<TransformGroupIndex>,
     monomorphic_legalize: Option<TransformGroupIndex>,