Headers containing only implementation details are better to stay in a
separate dir to indicate the user that they shouldn't be included.

Bug: chromium:1056170
Change-Id: I10f84ddf709b146396aadc820ec33bc6a49b2dac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2139585Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67050}