Restore `builtins.pathExists` behavior on broken symlinks
(cherry picked from commit d53c8901ef7f2033855dd99063522e3d56a19dab)
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note that this variant differs markedly from the source commit because
we haven't endured quite as much lazy trees.
Change-Id: I0facf282f21fe0db4134be5c65a8368c1b3a06fc
