Move getStagingRoot() to PathLocator

Problem/Motivation

So #3299093: Add validation that staging directory is writable necessitates that we be able to access the staging root even without a stage having been created. That means we gotta move getStagingRoot() to the path locator.

This is actually a good thing because it simplifies a bunch of stuff. It's a bad thing because the movement of that method has outsized implications on the rest of the code (which is why, indeed, it's a good idea to move it in the first place).

API changes

PathLocator will get a new getStagingRoot() method.