Cleaned up SkSL includes

This removes a bunch of includes from header files and replaces them
with forward declarations.

Change-Id: I14a766b84913817f2989437eb78760cec5f048e4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/460976
Reviewed-by: John Stiles <johnstiles@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
7 files changed