Core: Move DirAccess and FileAccess to core/io

File handling APIs are typically considered part of I/O, and we did have most
`FileAccess` implementations in `core/io` already.

modules/gdscript/gdscript_analyzer.cpp

@@ -31,10 +31,10 @@
 #include "gdscript_analyzer.h"
 
 #include "core/config/project_settings.h"
+#include "core/io/file_access.h"
 #include "core/io/resource_loader.h"
 #include "core/object/class_db.h"
 #include "core/object/script_language.h"
-#include "core/os/file_access.h"
 #include "core/templates/hash_map.h"
 #include "gdscript.h"
 #include "gdscript_utility_functions.h"