I see your point about keeping this out of the API, but I don't think the `filesystem` package is the right place in the long run and we will end up moving it later. The main utility of the class is as part of an SPI -- so we can change the API and help implementations along. Co-location tells implementers that this is something they should consider using, which we would lose by putting it in `filesystem`. I think our options should be to keep it where it is or to start a new package for this purpose.