Add presubmit check to detect accidental leakage of src/ into include/

Also add this to the style guide as it appears to not be explicitly
written what the purpose of our various folders are.

Change-Id: I768c00fb7fad928d6b339f08d0307c44484ae25c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/874677
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Kaylee Lubick <kjlubick@google.com>
2 files changed