client: catalog: fallback to metadata archives for catalog dump

client: catalog: fallback to metadata archives for catalog dump

Commit c0302805c "client: backup: conditionally write catalog for
file level backups" drops encoding of the dedicated catalog when
archives are encoded as split metadata/data archives with the
`change-detection-mode` set to `data` or `metadata`.

Since the catalog is not present anymore, fallback to use the pxar
metadata archives in the manifest (if present) for generating the
listing of contents in a compatible manner.

Signed-off-by: Christian Ebner <c.ebner@proxmox.com>