Store download URL for packages used by nuget_repo / nuget_archive

[WhoCalledInTheFleet]

There are NuGet repository implementations (for example that of JFrog Artifactory) that use different URL patterns for hosting NuGet packages:

https://ART_REPO/artifactory/api/nuget/nuget-all/Download/Some.Package.Name/1.0.0

Either at download time the rule should call FindPackagesById() and read the src attribute of the <content> element; Or we should have the package def (and also paket2bazel) store the full download URL instead of just version and sources.

[purkhusid]

I'm pretty sure that this is a bug in Artifactory since it does not seem to follow the convention that is outlined here https://learn.microsoft.com/en-us/nuget/api/overview

There is a workaround mentioned here: #379

I've been thinking that it might make sense to get rid of all the nuget URL handling and have just the full download URL instead. I don't have time to look into that currently so I don't expect that to be fixed very soon.