swh.loader.cran: Implement CRAN loader

Implement CRAN Loader using base package manager loader. Depends on D1744

---

Migrated from D1728 (view on Phabricator)

swh/loader/cran/loader.py

+from swh.loader.base.loader import BaseLoader
+from swh.loader.base.dowload import If_Modified_Since
+
+
+class CRANLoader(BaseLoader, If_Modified_Since):
+    loader_name = 'cran'
+    class_name = 'CRANLoader'
+
+    def convert_to_standard_format(**kwargs):
+        return {
+            'name': kwargs['name'],
+            'origin_url': kwargs['origin_url'],
+            'tarballs': kwargs['tarballs']
+        }