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_parse/src/lib.rs

@@ -20,7 +20,6 @@ use rustc_ast::{AttrItem, Attribute, MetaItem};
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::sync::Lrc;
 use rustc_errors::{Diagnostic, FatalError, Level, PResult};
-use rustc_fluent_macro::fluent_messages;
 use rustc_session::parse::ParseSess;
 use rustc_span::{FileName, SourceFile, Span};
 
@@ -36,7 +35,7 @@ pub mod validate_attr;
 
 mod errors;
 
-fluent_messages! { "../messages.ftl" }
+rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 // A bunch of utility functions of the form `parse_<thing>_from_<source>`
 // where <thing> includes crate, expr, item, stmt, tts, and one that