package.debian: Re-implement debian loader within the package loader mechanism

This introduces the debian package loader.

Contrary to other package loaders, a package can have multiple artifacts (per package version).

In those terms, we had to slightly adapt the swh.loader.package.loader.PackageLoader to demonstrate this. Thus the impacts on pypi, npm, deposit modules as well.

Related #2025 (closed) Related swh-core!91 (closed) Related swh-lister!114 (closed)

Test Plan

tox

---

Migrated from D2135 (view on Phabricator)

swh/loader/package/tests/common.py

@@ -54,8 +54,15 @@ def check_snapshot(expected_snapshot, storage):
     if snap is None:
         # display known snapshots instead
         from pprint import pprint
-        for snap_id, (_snap, branches) in storage._snapshots.items():
-            pprint(_snap.to_dict())
+        for snap_id, (_snap, _) in storage._snapshots.items():
+            snapd = _snap.to_dict()
+            snapd['id'] = hash_to_hex(snapd['id'])
+            branches = {
+                branch.decode('utf-8'): decode_target(target)
+                for branch, target in snapd['branches'].items()
+            }
+            snapd['branches'] = branches
+            pprint(snapd)
         raise AssertionError('Snapshot is not found')
 
     branches = {