refactor: allow relative symlinks that points out of the treeartifact

[thesayyn]

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Fixes https://github.com/bazelbuild/bazel/issues/20891

[thesayyn]

cc @tjgq @comius

[comius]

cc @tjgq @comius

Thanks. I believe @tjgq is more qualified reviewer in this case.

[tjgq]

df07d275ce17f14224af0f1a5862755a7db44397 touched some code around this area, so you'll have to rebase your PR.

[thesayyn]

All right. I'll get to it soon hopefully.

[thesayyn]

@tjgq dangling symlinks isn't seem to be supported anymore. is that expected? i get tests failing with "is a dangling symbolic link"

[thesayyn]

Also would it be possible to cherry pick this into 7.1.0? We'd like to start rolling this out with rules_oci as soon as we can.

[thesayyn]

@tjgq Ready for another pass.

[tjgq]

@bazel-io fork 7.1.0