migrate FSharp.Core.Microsoft.Signed to FSharp.Core

fix fsharp/fsharp/#352

These `nupkg`s will be identical, except their `id`s are `FSharp.Core`.
Oh, I understand now, you run "migrate.cmd" to get the files for the old packages.  

At least for the _latest_ version I think it would be better to start with a clean combined package specification that doesn't rely on fetching the contents of the old deprecated packages.