overlay: fix symlink-replacing-directory case

[tycho]

This fixes several image pulls with GitLab CE images, where a symlink replaced a directory path.

Before:

$ ./target/release/ocirender pull --image registry.gitlab.com/gitlab-org/build/cng/gitlab-toolbox-ce:v18.9.2 --output-squashfs x.squashfs
Error: merging layers into mksquashfs stdin

Caused by:
    0: mksquashfs failed (status=exit status: 1):
       FATAL ERROR: /srv/gitlab/log exists in the tar file as a non-directory, cannot add tar pathname srv/gitlab/log/.gitkeep!

    1: emitting srv/gitlab/public/-/emojis/4/alarm_clock.png
    2: failed to append entry: Broken pipe (os error 32)

After:

$ ./target/release/ocirender pull --image registry.gitlab.com/gitlab-org/build/cng/gitlab-toolbox-ce:v18.9.2 --output-squashfs x.squashfs
Done: x.squashfs
$ unsquashfs -lls -lln x.squashfs | grep log/gitl
lrwxrwxrwx 0/0                      15 2026-03-10 11:28 squashfs-root/srv/gitlab/log -> /var/log/gitlab
drwxrwxr-x 1000/1000                31 2026-03-10 10:51 squashfs-root/var/log/gitlab
-rw-rw-r-- 1000/1000                 0 2026-03-10 10:51 squashfs-root/var/log/gitlab/.gitkeep

All tests passing, and two new tests added.