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`.
Where does this "packages\FSharp.Core.4.3.0.0.Microsoft.Signed.3.0.0.0\lib" directory come from (and likewise below)?  It's not in the github repo. Don't you just want the full set of files here: https://github.com/fsharp/fsharp/blob/master/FSharp.Core.Nuget/FSharp.Core.Microsoft.Signed.3.0.nuspec#L25?