[includes] Enforce IWYU on base pure headers

Client changes:
 - http://ag/20588633

Change-Id: Ia084c22f7163a706ade45ee1e8ea4b9f89011313
Bug: skia:13052
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/610776
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Owners-Override: Kevin Lubick <kjlubick@google.com>
18 files changed