Do not rely on the existence of `config.py` while detecting modules

The existence of `SCsub` is checked instead. This file is required for
all modules, and prevents the build system to leave modules without
`config.py` undetected, leading to silently ignoring the module during
compilation.

(cherry picked from commit 12389b37363db541517c1c722e2e82294e92e251)

methods.py

@@ -149,7 +149,7 @@ def detect_modules(at_path):
 
 
 def is_module(path):
-    return os.path.isdir(path) and os.path.exists(path + "/config.py")
+    return os.path.isdir(path) and os.path.exists(os.path.join(path, "SCsub"))
 
 
 def write_modules(module_list):