restructured SkSL include files

Bug: skia:
Change-Id: I342de85d9602c953c8a8fd0f3891cd86bb5c35eb
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215614
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
6 files changed