Add a lister of all available packages in the npm registry

This diff adds a new lister in order to discover all packages available in the npm registry (https://replicate.npmjs.com/) and create loading tasks to ingest them into the archive.

At the time of writing, 839234 packages are registered in it. Next step, write a npm loader.

Related T1378 Closes #1380 (closed)

---

Migrated from D706 (view on Phabricator)

swh/lister/npm/tests/api_empty_response.json

+{
+  "total_rows": 839080,
+  "offset": 839080,
+  "rows": []
+}
\ No newline at end of file