Collect endpoint sorting ability and default ordering.

[tdruez]

In general, when multiple package entries are found in the PurlDB, we decided to use the most recent version for "package enriching" or data display in ScanCode.io and DejaCode.

When using the packages endpoint, one can use the sort=-version filter for ordering, but this is not available on the collect endpoint.

I think we should:

1. Add the ability to provide the sort parameter to the collect endpoint
2. Set a default order_by on the collect endpoint package QuerySet, to avoid returning a random order by the results. I would suggest always sorting by the most recent version, ie: -version, to be coherent with the data we care to display first.

[JonoYang]

@tdruez I've set ordering of the package results from collect to -version. Just wondering: should the sort parameter be a list of fields?

[tdruez]

@JonoYang We want consistency across all endpoints. The sort parameter on collect should work the same way it does on the packages endpoint.
https://github.com/nexB/purldb/blob/main/packagedb/api.py#L334

[JonoYang]

@tdruez I've merged #513 and I've deployed it to public instance of purldb. Let me know if there are more changes needed