Adapt code, test fixture and tests after the lister split its artifacts
data to artifacts and arch_metadata (see D8259)

Related T4233

Add 'aur' package loader

It seems that despite setting it in the 'except BaseException' block,
it is still occasionally undefined in the 'finally' block when
triggered by a SystemExit exception.

This should hopefully prevent UnboundLocalError from
being raised from the 'finally' block from now on

This fix an issue discovered while testing loader on docker environment,
error was (load_arch() got an unexpected keyword argument
'lister_name')

Related T4233

named args

This fix an issue discovered while testing loader on docker environment,
error was (load_crates() got an unexpected keyword argument 'lister_name')

Related T4104

They should use the same convention as Git.

Fetch Arch linux packages from lister discovered origins.
For each origin it get versions from extra_loader_arguments['artifacts']

Arch Linux package can comes as .xz or .zst file archive.
Support for .zst (Zstandard compression) has been requested with D7993.

Related to T4233

An artifact without time info can be provided in the artifacts list
parameter of the loader (for instance last modification date
is not available for tarballs coming from github releases).

That case was not handled by the archive loader wich was resulting
in loading error so add fix for it.

It will be used by the git loader to run a graph traversal on
received packfiles; and we want to monitor its runtime as a separate
metric.

It will be used by the git loader to compute metrics on the number
of objects ignored by the filtering proxy

The post_load method of a loader can raise an exception so we must
ensure to turn back the success variable to False in that case.

For instance, the subversion loader post_load checks that latest
exported revision is consistent with what the official subversion
client produces. If it is not an exception will be raised to set
the visit status to partial.

Related to T4236

The loader get 'artifacts' information through lister
'extra_loader_arguments'.

Related to T4104

artifacts list sent by the lister is not guaranteed to be sorted
by artifact time so the last element is not necessarily the most
recent artifact.

So ensure to use the most recent artifact as default version by
sorting the artifacts by their ISO 8601 date representation
(designed for lexicographical sort). If a date failed to be
parsed by the lister (edge case but can happen), the time value
will be an empty string so the sort operation will not end up
with error.

Related to T3874

It makes the whole swh documentation build fails as relative path to the
JSON file is not the same when building swh-docs, plus sphinx generates
a link to the Crates API URL enabling to browse the JSON data.

Maven lister has been updated to create one origin per package and
group all its versions in it as releases.

Previously a maven origin URL corresponded to a jar archive URL
while now it corresponds to the page where all versions of a
package can be found.

So update loader implementation to reflect this change and fix
jar archives download.

Also update tests after this change and refactor them a bit,
notably by using fixtures instead of hardcoded tests data.

Related to T3874

Recent changes in swh-scheduler add new parameters to the celery tasks
produced from swh.scheduler.model.ListedOrigin instances.

So ensure to handle any new parameters by not hardcoding the expected
ones in task signatures.

Remove unsafe use of unnamed task parameters.

Add new tests checking task parameters produced from ListedOrigin
instances do no raise error when attempting to create a package loader.

Related to T4187

So it is available to concrete loaders.

The Git loader will use it to incrementally load new origins by fetching
the last snapshot of a parent, instead of starting from scratch.

This uses setuptools entrypoints to allow metadata fetchers to
make themselves discoverable without explicitly depending on them.

This commit also lifts the generic metadata loading from the base
PackageLoader to BaseLoader

Some archive integrity checks were failing after downloads as computed
md5sums was compared against empty ones.

Consequently some versions stored as branches were discarded from the
produced snapshot of a debian package.

1. Pass **kwargs to the base loader, instead of repeating the args
2. Remove redundant attribute initialization

That hook can be frustrating as it can discard a long commit message
if it finds a typo in it so better removing it.

Loaders only use prepare_origin_visit to set the origin and visit date,
so it is simpler to do it in `BaseLoader.__init__`.

For consistency with VCS loaders.

A future commit will introduce more code shared between package and VCS
loaders, and will need to use this attribute.

Prior to this commit, the test data were in the wrong format. They were passing along an
url within the artifact parameter which is not provided by the lister.

This fixes the inconsistency.

Related to T3874