crates: Improve loader implementation
Add incremental support based on sha256 EXTID.
Use looseversion.LooseVersion2 to parse crate versions.
Manage release date for each versions of a package.
Fetch extrinsic metadata for a crate version from crates Web API.
Adapt test dataset and add incremental test cases.
Superseeds !427 (closed)
Related to swh-lister!532 (merged)
Related to swh/meta#4104
Edited by Antoine Lambert