Consider symlinks when normalizing output locations while processing buildpack layers

In addition to the changes made for #50141, we can harden things further by also considering symlinks. Change proposed by @ghosttT0:

if (entry.isSymbolicLink() || entry.isLink()) {
    String linkName = entry.getLinkName();
    Path resolvedLink = basePath.resolve(linkName).toAbsolutePath().normalize();
    Assert.state(!linkName.startsWith("/") && resolvedLink.startsWith(basePath.toAbsolutePath()),
        () -> "Link target '%s' in entry '%s' cannot point outside of '%s'"
            .formatted(linkName, entryName, basePath));
}

The issue does not name files or tests; start by tracing the buildpack-layer code that normalizes output locations while processing layers. Check how symbolic and hard links are represented there, then verify that link targets remain within the layer output base and add or run focused coverage for links pointing outside it.