Refactor `project_dataroot` method in Project.rb

[Bubballoo3]

It seems like there are several different attempts to handle the case of a relative directory being passed, and it is surprising that it hasn't caused more problems. Once the major changes are merged from #4792 and #4840, we should pivot the approach so that we handle relative directories in Project#initialize, allowing us to trust the absolute-ness of the :directory attribute.

We can also use that as an opportunity to clean up the return types of each of those methods, as we currently have a mix of string and pathname objects.