perf: packages: optimize fetching depends and required_by relations

Avoid unnecessary queries for the total count of both, the dependencies
of a package as well as its reverse (required by) relations whenever
possible, which has a quite high cost especially for doing nothing that
would actually be required in most cases as often packages have less
depends and required by than the max listing of 20.

Avoid fetching the total count if:
- The count of the fetched elements is below the max listing
- All elements are requested to be fetched