snapshot.h is intended to be the public header for the snapshot
component, and not the right place for private declarations. This moves
them into a new SnapshotImpl class in snapshot.cc (previously named
snapshot-common.cc).

Bug: v8:10416
Change-Id: If34ad8d6e189050686942488fb8e99c3d310beee
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2144062
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67279}