Use Coreclr transport packages (dotnet/corefx#37163)

Use Coreclr transport packages (dotnet/corefx#37163)

* Replace CoreCLR package with transport package

This allows us to remove the manual download step

* Remove IL and NI folders, use IL in debug or when specified

I also simplified some of these targets.   NuGet automatically includes all files in native,
so additional inclusion was unnecessary.

* Disable IL CoreLib swap for UAP10.0.16299 and AOT

Commit migrated from https://github.com/dotnet/corefx/commit/8c7afe67ee2e9bd21a4412265b66f875cfafaede