Add WindowsDesktop targeting pack nupkg, refactor pkg projects (dotnet/core-setup...

Add WindowsDesktop targeting pack nupkg, refactor pkg projects (dotnet/core-setup#5214)

* Refactor pkg projects

Instead of '.builds' projects, get Project items by calling a target in '.pkgproj' projects.

Move more common props/targets together.

* Add WindowsDesktop targeting pack nupkg

* Stop placing all files in data/ dir, only txt

* Change Pack nupkg type to DotnetPlatform

* Rename framework targets files

* Remove source redist: obsolete

* Require version files to be found and exist

Explicitly check if they exist to throw a clear error. The ReadAllText call would catch this, but it's verbose and doesn't say what the item's name is.

* DetermineRuntimeDependencies is not an override

* Remove package-specific asset exclusion

Use ExcludeAssets=All for CoreRT package to restore it but exclude Windows.winmd from redist.

Commit migrated from https://github.com/dotnet/core-setup/commit/2f91bd28d0c77eb72275f46ddd7cfc9f10196ba7