Add additional matrix split to nuspec

Update targets file to only import if matching $Platform
When building cef.redist nuget packages update cef.redist.targets to include a unique target name for 32/64bit otherwise they don't seem to coexist happily
Remove item group entries, don't believe it's relevant anymore
The build script will change the target name to either `CefRedistCopyDllPak32` or `CefRedistCopyDllPak64` so the can co-exist. 

The condition on the target means it's only executed when it contains a folder matching the $(Platform) variable.
e.g. If it's a x86 build, and the package contains an x64 folder the target won't execute, and vice-verse the target will execute if they match.