swh.loader.cran: Implement CRAN loader

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

---

Migrated from D1728 (view on Phabricator)

Build is green See https://jenkins.softwareheritage.org/job/DLDBASE/job/tox/124/ for more details.

This is the proposed implementation of CRAN Loader using base loader(!368 (closed)) This implementation requires some small refactorings in cran lister

Merge request was abandoned

closed

mentioned in merge request !368 (closed)

mentioned in issue swh/meta#2029 (closed)