Serialization: Snippet folders and Collections collide on dump because of missing namespace separation

[flamber]

Describe the bug
If there's a Collection and a Snippet Folder with the same name, then only one of them is created, and the order (ID) in the database will determine which will be overridden, and that causes questions (or potentially snippets) not to be transferred.

To Reproduce (very important with the order of when items are created)

1. Create a collection called "Marketing"
2. Create a question and save it inside of "Marketing"-collection
3. Create a snippet folder called "Marketing"
4. Create a snippet and save it inside of "Marketing"-folder
5. Serialization dump - notice that the collection-yaml for "Marketing" is namespace: snippets and it has the question-yaml inside of it. There's no snippet-yaml, but that's Serialization: Snippets are not transferred correctly, leading to incorrect references and broken queries #14909.
6. Serialization load will give the following error, which is correct, but there was no errors/warnings during dump that different items was overwriting each other:
   `ERROR serialization.upsert :: Error inserting metabase.models.card.CardInstance@6b213571 "My Question in Marketing-collection": A Card can only go in Collections in the "default" namespace.

Expected behavior
Not sure if serialization-dump/collections/root/collections should have a sub-directory for each namespace, or if snippet folders should have it's own parent directory like serialization-dump/collections/root/snippet-folders, but they shouldn't overlap.

Information about your Metabase Installation:
Only tested 1.37.9 because #14907 made it even more confusing.

Additional context
Should probably be fixed together with #14909, since snippets are not transferred at all, and this just leads to more confusion.

[jeff303]

Commit 5 under #15858 should fix this one.

[flamber]

Fixed by #15858 - will be part of 1.39.3