Prior to this commit, the origin url was targeting solely one versioned
artifact. This adaptation allows to have a canonical url as origin to which we
associate a list of artifacts to ingest (symmetrically as what is done for
other package loaders).

Related to D2524

This is setup-ed through configuration using the proxy storage.

Use directly self.storage.origin_* endpoints

The only loader initially using this was the old loader deposit implementation.
The new one implements it on its own.

This commit fixes unicode error raised during DESCRIPTION file parsing.

Fix https://sentry.softwareheritage.org/share/issue/6c4750b56a114817bb088ec6e4965faf/

Related T2026

Drop the following computation hashes:
- length (which was read from the content-length header, and sometimes it was
  wrongly read because of some missing header in the initial request)
- sha1_git
- blake2s256

Add tests around it

DVCS loader initializes their visit_type attribute as class attribute.
This was set to None as class attribute happens before constructor.

Prior to send data to storage

The old buffer and filter behavior must be installed through configuration now.
It's now delegated to specific buffer and filter storages.

Beware, this broke some dvcs loaders (at least svn) relying on some removed
api.

2 subcommands:
- one to load
- one to instrospect the loader's needed arguments

Related to T2134

Related to T2134

Related to T2134

Related to T2134

Related to T2134

Related to T2134