Targeting pack support for .NET Core 3.0

For .NET Core 3.0, we expect to separate targeting packs, which contain the assets needed at build time when targeting .NET Core 3.0 (for example, reference assemblies and intellisense documentation) from “runtime packs”, which will include the platform-specific runtime components for .NET Core 3.

This issue tracks the work required in order to support targeting packs. This may include:

• Figure out what the layout of targeting packs will be
• Create the targeting packs for each supported shared framework (as NuGet packages and msi, apt-get, etc installation packages)
  • .NET Core
  • ASP.NET Core
  • Microsoft.WindowsDesktop.App
  • .NET Standard (this is not a shared framework, but the targeting pack should work in a similar way)
• Bundle targeting packs in core-sdk
• Consume targeting packs at build time - resolve targeting packs that ship with the SDK, add “download only” package reference if need be
• Supply RID graph for .NET Core 3.0 SDK
• Trim earlier versions of platform packages from NuGet graph
• Support explicit targeting pack acquisition (ie via a command such as dotnet targeting-pack install netcoreapp3.0)
• Support build logic which was previously in the package closure of Microsoft.AspNetCore.App

Finer-grained issues for these tasks will be filed as necessary

[dasMulli]

Ideally, this shouldn’t break usages of

MetadataReference[] _ref = 
    DependencyContext.Default.CompileLibraries
    .First(cl => cl.Name == "Microsoft.NETCore.App")
    .ResolveReferencePaths()
    .Select(asm => MetadataReference.CreateFromFile(asm))
    .ToArray();

when using PreserveCompilationContext=true

It should only need an update to the dependency model NuGet package.

[dasMulli]

Same as for https://github.com/dotnet/cli/issues/10007, please consider:

• Clean-up story
• Location and permission configuration. This may be important for build scripts that may not have superuser / elevated permissions and would need to use a custom folder (dotnet targeting-pack install net472 -o ./targeting-packs; dotnet build foo/foo.csproj --targeting-packs ./targeting-pakcs)