Crates.io: Add last_update for each version of a crate
In order to reduce http api call amount made by the loader, download a
crates.io database dump, and parse its csv files to get a last_update
value for each versions of a Crate.
Those values are sent to the loader through extra_loader_arguments
'crates_metadata'.
'artifacts' and 'crates_metadata' now uses "version" as key.