[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/gen_legalizer.rs

@@ -577,7 +577,7 @@ fn gen_isa(
 }
 
 /// Generate the legalizer files.
-pub fn generate(
+pub(crate) fn generate(
     isas: &Vec<TargetIsa>,
     format_registry: &FormatRegistry,
     transform_groups: &TransformGroups,