Once the folder path is generated, it is guaranteed to exist and to be a directory.
The path is based on 3 levels of depth: the library name, a namespace and a
subfolder. Those 3 levels grants flexibility while allowing `huggingface_hub` to
expect folders when scanning/deleting parts of the assets cache. Within a library,
it is expected that all namespaces share the same subset of subfolder names but this
is not a mandatory rule. The downstream library has then full control on which file