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_driver_impl/src/lib.rs

@@ -30,7 +30,6 @@ use rustc_errors::registry::{InvalidErrorCode, Registry};
 use rustc_errors::{markdown, ColorConfig};
 use rustc_errors::{ErrorGuaranteed, Handler, PResult};
 use rustc_feature::find_gated_cfg;
-use rustc_fluent_macro::fluent_messages;
 use rustc_interface::util::{self, collect_crate_types, get_codegen_backend};
 use rustc_interface::{interface, Queries};
 use rustc_lint::unerased_lint_store;
@@ -102,7 +101,7 @@ use crate::session_diagnostics::{
     RLinkWrongFileType, RlinkNotAFile, RlinkUnableToRead,
 };
 
-fluent_messages! { "../messages.ftl" }
+rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 pub static DEFAULT_LOCALE_RESOURCES: &[&str] = &[
     // tidy-alphabetical-start