Use rustc_fluent_macro::fluent_messages! directly.

Currently we always do this:
```
use rustc_fluent_macro::fluent_messages;
...
fluent_messages! { "./example.ftl" }
```
But there is no need, we can just do this everywhere:
```
rustc_fluent_macro::fluent_messages! { "./example.ftl" }
```
which is shorter.

compiler/rustc_errors/src/lib.rs

@@ -41,7 +41,6 @@ pub use rustc_error_messages::{
     fallback_fluent_bundle, fluent_bundle, DelayDm, DiagnosticMessage, FluentBundle,
     LanguageIdentifier, LazyFallbackBundle, MultiSpan, SpanLabel, SubdiagnosticMessage,
 };
-use rustc_fluent_macro::fluent_messages;
 pub use rustc_lint_defs::{pluralize, Applicability};
 use rustc_span::source_map::SourceMap;
 pub use rustc_span::ErrorGuaranteed;
@@ -82,7 +81,7 @@ pub use snippet::Style;
 pub type PErr<'a> = DiagnosticBuilder<'a, ErrorGuaranteed>;
 pub type PResult<'a, T> = Result<T, PErr<'a>>;
 
-fluent_messages! { "../messages.ftl" }
+rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 // `PResult` is used a lot. Make sure it doesn't unintentionally get bigger.
 // (See also the comment on `DiagnosticBuilderInner`'s `diagnostic` field.)