export names only once (#3392)

names declared to be exported at the point of introduction into a namespace *must not* be (re-)declared as exported later in the TU, e.g. when they are redeclared, defined, or specialized. [module.export]/6

Drive-by fix found during module testing: add a missing `detail::` name qualification

include/fmt/ostream.h

@@ -111,7 +111,7 @@ struct basic_ostream_formatter : formatter<basic_string_view<Char>, Char> {
   auto format(const T& value, basic_format_context<OutputIt, Char>& ctx) const
       -> OutputIt {
     auto buffer = basic_memory_buffer<Char>();
-    format_value(buffer, value, ctx.locale());
+    detail::format_value(buffer, value, ctx.locale());
     return formatter<basic_string_view<Char>, Char>::format(
         {buffer.data(), buffer.size()}, ctx);
   }