ExcludeFn returns a filesystem identical to fsys excluding paths for which hide(path) returns true.
Hiding a directory hides all contained subdirectories and files.
ExcludeFn panics if hide(".") returns true.
ExcludePaths returns a filesystem identical to fsys excluding all paths in hide.
Hiding a directory hides all contained subdirectories and files.
ExcludePaths panics if hide includes the path ".".