refactor: mark module_utils modules as private (#782)

##### SUMMARY

All `module_utils` are now marked as **private**. None of the modules
were intended for public use.

Similar to
https://togithub.com/ansible-collections/community.general/issues/11312

plugins/module_utils/_storage_box_snapshot.py

@@ -0,0 +1,24 @@
+# Note that this module util is **PRIVATE** to the collection. It can have breaking changes at any time.
+# Do not use this from other collections or standalone plugins/modules!
+
+from __future__ import annotations
+
+from ._vendor.hcloud.storage_boxes import (
+    BoundStorageBoxSnapshot,
+)
+
+
+def prepare_result(o: BoundStorageBoxSnapshot):
+    return {
+        "storage_box": o.storage_box.id,
+        "id": o.id,
+        "name": o.name,
+        "description": o.description,
+        "labels": o.labels,
+        "stats": {
+            "size": o.stats.size,
+            "size_filesystem": o.stats.size_filesystem,
+        },
+        "is_automatic": o.is_automatic,
+        "created": o.created.isoformat(),
+    }