errors: move translation logic into module

Just moving code around so that triagebot can ping relevant parties when
translation logic is modified.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_errors/src/lib.rs

@@ -58,6 +58,7 @@ mod lock;
 pub mod registry;
 mod snippet;
 mod styled_buffer;
+pub mod translation;
 
 pub use snippet::Style;