Trim earlier versions of platform packages from NuGet graph

[dsplaisted]

With .NET Core 3.0, we will be using targeting packs to deliver reference assemblies for the framework. These targeting packs will not be represented as NuGet packages in the package graph in the way that Microsoft.NETCore.App, Microsoft.AspNetCore.App, and NETStandard.Library currently are.

When a .NET Core 3 project depends on a package that has a dependency on one of these older "platform" packages, we want that platform package and its dependencies to be ignored in the NuGet graph, as those APIs are supplied by the platform via other mechanisms (targeting packs at build time and runtime packs for self-contained deployments).

We've thought of two possible ways to exclude the platform packages and dependencies from the graph:

Use empty platform packages

One option is for us to produce empty 3.0 versions of these platform packages, and implicitly reference them from .NET Core 3.0 projects. This would prevent the previous versions of the packages, as well as their dependencies, from being used in the graph.

However, these empty 3.0 packages would be confusing to see in the list of installed packages in the package manager UI. So if we go with the "empty package" option, we'd like a way to entirely exclude these packages from the NuGet UI.

This option also has the downside that it requires us to ship these empty packages in the NuGetFallbackFolder in the SDK, which we'd like to get rid of.

Pass "supplied by platform" packages to NuGet

This would involve a feature in NuGet: NuGet/Home#7344

We would pass the list of platform packages that should be ignored to the restore operation, and it would ignore those packages if it encountered them in the graph walk.