Only copy files from nuget packages that the project actually depends on

To work around an issue with nuget don't use absolute path for its hintpath elements in the csproj files we download all nuget packages to the same folder (and not under the solution's folder).
We also use mixed architectures with different nuget packages for x86 and x64. When Nuget.Output copies all output folders we get different results depending on which nuget packages that are downloaded at the moment.

This patch will read the project's package.config file and only copy the files the project actually depends on.

The new target FilterPackages will unpack the nuget spec files and the old UnzipNuspec target is gone.