Remove need for NuGet.exe download

As discussed in https://github.com/dotnet/aspnetcore/pull/23786#discussion_r451864623, we have an opportunity to remove use of NuGet.exe when wrapping our shared framework and targeting packs into `*.nupkg` files. Separate `*.csproj` projects that depend only on the SharedFramework.wixproj or TargetingPack.wixproj could work like other C# projects with custom `*.nuspec` files. It would mean two more projects and four more project invocations (one for each Windows `$(Platform)` per project) but removes a download.

Note that since the new C# projects would depend only on WiX projects, they'd need to run in desktop `msbuild`. Complicating things with a switch back to `dotnet msbuild` and discovery of the `*.msi` files et cetera doesn't seem worthwhile. That is, let's do this as long as it's a simplification.

(Putting this on the aspnet-build project and not the DoI one because this infrastructure directly affects few devs.)

/cc @dotnet/aspnet-build