[iOS] Move i/c/b/photos/ to i/c/b/photos/model/ subfolder

This CL is the first step to move the photos model folder to an actual
subfolder named "model". Since the content of that folder has references
outside of this repo, some files have to stay outside of "model/" for
the moment.

Bug: 1498935
Change-Id: I19b7886eb74d012cc6739c63a665ddbc93fa9635
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5001468
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Quentin Pubert <qpubert@google.com>
Cr-Commit-Position: refs/heads/main@{#1219369}