Create targeting pack for Microsoft.WindowsDesktop.App #3399

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. See dotnet/cli#10006, and dotnet/designs#50

We will have targeting pack for each shared framework (.NET Core, ASP.NET Core, and WindowsDesktop), as well as for .NET Standard. This issue tracks the work required for the WindowsDesktop shared framework.

The work required includes:

• A new NuGet package including the targeting pack assets. This should include the reference assemblies, intellisense documentation, and platform manifest file. The current straw man proposal for the package ID is Microsoft.NETCore.App.Ref
• Installers for each supported installer format (msi, deb, pkg, and rpm), which lay out the targeting pack files under <DOTNET_ROOT>/packs/<PackageId>/version. These installers will be bundled or depended on by the corresponding .NET Core SDK installers.

Some of the details (such as package layout, names, etc) are still under design and may change slightly.