Do *not* export namespace detail

Introduce `FMT_BEGIN_DETAIL_NAMESPACE` and `FMT_END_DETAIL_NAMESPACE` for `namespace detail` sections embedded in that part of the code that contains all declarations that are exported from the module, i.e. which is enclosed by `FMT_MODULE_EXPORT_BEGIN` and `FMT_MODULE_EXPORT_END`. Given a correct implementation of C++20 modules, neither the name `fmt::detail` nor any of its contents will become visible outside of the module.

src/fmt.cc

@@ -70,6 +70,12 @@ export module fmt;
 #define FMT_MODULE_EXPORT export
 #define FMT_MODULE_EXPORT_BEGIN export {
 #define FMT_MODULE_EXPORT_END }
+#define FMT_BEGIN_DETAIL_NAMESPACE \
+  }                                \
+  namespace detail {
+#define FMT_END_DETAIL_NAMESPACE \
+  }                              \
+  export {
 
 // all library-provided declarations and definitions
 // must be in the module purview to be exported