Move src-internal repo mapping from settings_internal to settings/internal.

This will allow access to files in this repo from settings/BUILD.gn,
which is necessary in followup https://crrev.com/c/5218772.

Bug: 1467623, b/319118045
Change-Id: I0ef19b3812e58cfb743b7b51cf9352bdd1b4e613
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5225481
Reviewed-by: Ted Choc <tedchoc@chromium.org>
Reviewed-by: John Lee <johntlee@chromium.org>
Commit-Queue: Rusty Sammon <sammon@google.com>
Reviewed-by: Alan Zhao <ayzhao@google.com>
Cr-Commit-Position: refs/heads/main@{#1251582}