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

mentioned in merge request !427 (closed)

changed the description

Jenkins job DLDBASE/gitlab-builds #339 succeeded .
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approved this merge request

I suppose it's ok

added 5 commits

• 0c382896...06643984 - 4 commits from branch swh/devel:master
• 89659947 - crates: Improve loader implementation

Compare with previous version

Jenkins job DLDBASE/gitlab-builds #346 succeeded .
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merged manually