Similar to cl/9715, this makes the validation checks more granular,
introducing a Validate on all *Node.
A check for symlink targets is added too.
Once merged, it can also be used from tvix/store/protos.
Change-Id: I0909a89fadcd74b74ef0c9a8a1f22658fccc83b0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9716
Reviewed-by: Connor Brewster <cbrewster@hey.com>
Tested-by: BuildkiteCI