cran and nixguix: Make known_artifact_to_extid static

Like the PyPI and NPM loader.

It allows scripts to use the method without creating a loader instance.

swh/loader/package/cran/loader.py

@@ -88,7 +88,8 @@ class CRANLoader(PackageLoader[CRANPackageInfo]):
             if version == p_info.version:
                 yield release_name(version), p_info
 
-    def known_artifact_to_extid(self, known_artifact: Dict) -> Optional[PartialExtID]:
+    @staticmethod
+    def known_artifact_to_extid(known_artifact: Dict) -> Optional[PartialExtID]:
         return CRANPackageInfo.from_metadata(known_artifact["extrinsic"]["raw"]).extid()
 
     def build_revision(

swh/loader/package/nixguix/loader.py

@@ -155,7 +155,8 @@ class NixGuixLoader(PackageLoader[NixGuixPackageInfo]):
             ret[revision.id] = revision.metadata
         return ret
 
-    def known_artifact_to_extid(self, known_artifact: Dict) -> Optional[PartialExtID]:
+    @staticmethod
+    def known_artifact_to_extid(known_artifact: Dict) -> Optional[PartialExtID]:
         try:
             value = known_artifact["extrinsic"]["raw"]["integrity"].encode("ascii")
         except KeyError as e: