Fix fs.WithSymlink lack of symmetry

`fs.WithSymlink`, when used with a Dir or File creates a link with an absolute path, when used with a Manifest it expects a relative path.

I think both cases could be useful, but having different behaviour in the same function is a problem. I think this function can be deprecated and replaced with: `fs.WithRelativeSymlink` and `fs.WithAbsoluteSymlink` which have consistent behaviour for all types of `Path`.