issue #10763 CREATE_SUBDIRS and C++20 modules: Doxygen doesn't create…

… directories

The `module_` should be part of the (file) name not prepended to the directory name.

src/moduledef.cpp

@@ -137,7 +137,7 @@ class ModuleDefImpl : public DefinitionMixin<ModuleDef>
 
 QCString ModuleDefImpl::getOutputFileBase() const
 {
-  return "module_"+convertNameToFile(name());
+  return convertNameToFile("module_" + name());
 }
 
 QCString ModuleDefImpl::qualifiedName() const