Fix issues with EnumerateVersionsAsync not handling 404s

[jpinz]

I noticed when implementing version 0.1.346 of OSSGadget that our unit tests for package versions that don't exist would fail, as
var packageVersionExistence = await projectManager.DetailedPackageVersionExistsAsync(purl); would throw an HttpRequestException where the StatusCode is HttpStatusCode.NotFound.
I would expect this case to return a PackageVersionNotFound instance of IPackageExistence instead of throwing a 404.

This PR adds:

catch (HttpRequestException ex) when (ex.StatusCode == HttpStatusCode.NotFound)
{
    Logger.Debug("Unable to enumerate versions (404): {0}", ex.Message);
    return Array.Empty<string>();
}

to each ProjectManager's implementation of EnumerateVersionsAsync.