in `preVisitDirectory`, `FileInstaller` uses `Path::relativize` incorrectly:
{code}
         public FileVisitResult preVisitDirectory(Path file,
                 BasicFileAttributes attrs) throws IOException {
            Path relativePath = file.relativize(copyFrom);
            ...
{code}

where `copyFrom` is the directory which contains `file`, and `Path::relativize` returns a relative path for an argument against receiver, not the other way around.