expose dir/file determiner

[sellout]

Functions like listDirRel figure out whether an entry is a directory or file and return the paritioned lists. However, I have path listings (with mixed files & dirs) that come from somewhere other than getDirectoryContents, so I can’t rely on that. Instead, something like

parseExistingPath :: MonadIO m => FilePath -> m (Either (SomePath Dir) (SomePath File))

(and maybe parseExistingAbsPath / parseExistingRelPath) would let me provide listDir-like functionality across other inputs.

And listDirRel itself should just use parseExistingAbsPath internally.