Collect endpoint sorting ability and default ordering.

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.