Rename io.delta.kernel.spark package

[TimothyW553]

The current package naming for the kernel-spark module uses io.delta.kernel.spark.*, which is misleading. This suggests the code is part of the core Delta Kernel library, when it's actually Spark integration code that uses Kernel.

Problem

• Package io.delta.kernel.spark.* implies this is Kernel code (io.delta.kernel.* should be reserved for core Kernel APIs)
• The module name kernel-spark has similar confusion