converters: Replace '/' with '_' in directory entries
'/' is very rare, but it is rejected by swh.model, because it would cause issues in other places (eg. cannot be accessed via the web UI or web API)
This applies the same transformation that T4644#97834 is going to apply on the 3 existing trees in the archive.
Depends on !117 (closed)
Closes #4660 (closed)
Migrated from D8776 (view on Phabricator)