loader.npm: If no upload time provided, use artifact's mtime if provided
Instead of failing the ingestion, fallback to use the artifact's mtime (if provided).
Note that most origins have their upload 'time' provided. In the error referenced [1], the origins sampled there show that they have their 'mtime' referenced instead of the 'time'.
Also, if no upload time is provided at all, skip the artifacts altogether. This allows ingestion of further artifacts for the same origin instead of failing completely the origin ingestion.
Test Plan
tox
Migrated from D2597 (view on Phabricator)