Cleans up the file structure to be more similar to that of the SVM
and also makes it possible to build kernels with OSL support, but
without having to include SVM support.
- kernel/osl/shader.cpp was merged into kernel/osl/closures.cpp and kernel/osl/globals.cpp
- kernel/osl/shader.h was renamed to kernel/osl/osl.h
This patch was split from D15902.