Affected dependents counts

[andrew]

Would be great to show a count of how many packages and repositories are potentially affected by an advisory.

This requires working out which versions of a package are affected, then counting all the packages/repos that list that package as a dependency within an affected version.

For packages this could inclucde ranges, for repositories it will mostly be looking for exact versions in lockfiles.

This will require new endpoints for both repos and packages services with filters on requirements.