Avoid need for {D,Subd}iagnosticMessage imports.

The `fluent_messages!` macro produces uses of
`crate::{D,Subd}iagnosticMessage`, which means that every crate using
the macro must have this import:
```
use rustc_errors::{DiagnosticMessage, SubdiagnosticMessage};
```

This commit changes the macro to instead use
`rustc_errors::{D,Subd}iagnosticMessage`, which avoids the need for the
imports.

compiler/rustc_builtin_macros/src/lib.rs

@@ -23,7 +23,6 @@ extern crate tracing;
 
 use crate::deriving::*;
 
-use rustc_errors::{DiagnosticMessage, SubdiagnosticMessage};
 use rustc_expand::base::{MacroExpanderFn, ResolverExpand, SyntaxExtensionKind};
 use rustc_expand::proc_macro::BangProcMacro;
 use rustc_fluent_macro::fluent_messages;