pubdev: Modify origin URL and retrieve package last update
Use https://pub.dev/packages/{pkgname}
as origin URL for a package
instead of https://pub.dev/api/packages/{pkgname}
.
Fetch package versions info directly in the lister in order to compute a last update date to send to scheduler datatabase.
Related to T4465
Migrated from D8354 (view on Phabricator)