Reuse existing elements and allow to import subfolders

Refs https://redmine.teklia.com/issues/4933

The ALTO import always creates new elements, whereas this corpus (FoNDUE-HTR and Gallicorpora folders) already has imported images with the right hierarchy (and the right names). We'd like to be able to re-use these elements directly and avoid creating new ones.

I was able to reuse element and import:

• a folder with sub-subfolders (demo no more available sorry) (with worker run)
• a folder with subfolder (demo)
• folder with pages (demo)

I was also able to create new elements by using this folder as parent (demo)

I haven't tested the gallica command but this command seems already broken

Commands

# Folder with sub-subfolder
arkindex -p preprod upload alto \
  . \
  --iiif-base-url "https://europe-gamma.iiif.teklia.com/iiif/2/public%2Fhtr-united%2F" \
  --parent-id a68d6ffe-a0b4-4d77-b96e-605907d25e32 \
  --create-types \
  --worker-run-id 88ed2f8d-bc7e-4594-8b9a-a2f246cfcef6

# Folder with subfolder
arkindex -p preprod upload alto \
  FONDUE-FR-PRINT-17 \
  --iiif-base-url "https://europe-gamma.iiif.teklia.com/iiif/2/public%2Fhtr-united%2F" \
  --parent-id c053c070-0c3d-4a9d-98b5-a47f17c32e37 \
  --create-types

# Folder with pages
arkindex -p preprod upload alto \
  HTR-imprime-18e-siecle/bpt6k1281160s/ \
  --iiif-base-url "https://europe-gamma.iiif.teklia.com/iiif/2/public%2Fhtr-united%2F" \
  --parent-id 0ae1e3cd-1eb3-4db3-a67a-8afccfc65753 \
  --create-types