npm: Explicitely retrieve the revision date from extrinsic metadata

No date is available in intrinsic metadata, so we retrieve it from the API
metadata, using the version number that the API claims this package has.

This fixes the current loading which fails when this scenario occurs.

Note: This matches what was done in the prior loader-npm version [1]

[1] https://forge.softwareheritage.org/source/swh-loader-npm/browse/master/swh/loader/npm/client.py$84-91