[windows] better implementation for NuGet files

[jonathanpeppers]

My previous implementation of Windows/NuGet had some issues:

• The <Copy /> tasks can cause double writes (only a perf issue)
• If using libZipSharp.csproj as a <ProjectReference />, the NuGet dlls are not copied to the other project's output directory
• I was using the <Touch /> task to create an empty file

I now have alot more MSBuild knowledge, changes include:

• Just make $(DllConfigOutputFile) conditional, we can remove the _UpdateDllConfigNotUnix target
• Use <Content /> and CopyToOutputDirectory to fix <ProjectReference /> in other projects

Successful Windows build on AppVeyor here.