Improve some names.

Lots of vectors of messages called `message` or `msg`. This commit
pluralizes them.

Note that `emit_message_default` and `emit_messages_default` both
already existed, and both process a vector, so I renamed the former
`emit_messages_default_inner` because it's called by the latter.

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -1384,7 +1384,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     if has_custom_message {
                         err.note(msg);
                     } else {
-                        err.message =
+                        err.messages =
                             vec![(rustc_errors::DiagnosticMessage::from(msg), Style::NoStyle)];
                     }
                     let mut file = None;