This allows to have a more generic "tar" loader (in swh-loader-core
repo, branch package-loader so far).
As proposed in D2145 proposition:
```
As a first draft, i'd say the contract is, provide:
- an origin url
- a list of artifacts: for each of those package artifacts, an url is needed, a
last modification date, at least one hash, and a version
(D2025#inline-14210).
```