Send package artifact checksums to loaders when info is available

In listers collecting artifacts for each package to load, add artifacts
checksums, when that info is available, in parameters sent to loaders
in order to check downloaded artifact integrity.

swh/lister/arch/lister.py

@@ -453,6 +453,14 @@ class ArchLister(StatelessLister[ArchListerPage]):
                         "length": version["length"],
                     }
                 )
+                if version["version"] == origin["version"]:
+                    artifacts[-1]["checksums"] = {
+                        "md5": origin["data"]["md5sum"],
+                        "sha256": origin["data"]["sha256sum"],
+                    }
+                else:
+                    artifacts[-1]["checksums"] = {"length": version["length"]}
+
                 arch_metadata.append(
                     {
                         "version": version["version"],