Split FilePath into DirPath and FilePath

Mmmm, not sure I understand how it all works then. In an example api:
```
def sizeOfDir(dir: DirPath): ResultT[IO, Int] = ???
```

How could I implement this in a way that a user could specify "tmp" in the CWD or "/tmp"? 

As long as that is really clear, I am ok with it.