chore: split Path into AbsolutePath and RelativePath

[DavePearce]

The goal here is to make it clear when an absolute path is being used. There are some challenges in making this work, but largely it should be possible. There is also one potential benefit from this as well:

• (virtual / non-virtual components). For an absolute path, it would be useful to distinguish the "virtual" segments from the non-virtual segments. This would allow us to generate a valid context directly from an absolute path, rather than carrying around the context separately.

From a terminology perspective, we could call them: static vs dynamic or outer vs inner, etc. They do remind me somehow of Java inner classes.

For a relative path, separating the virtual and non-virtual components perhaps doesn't matter since we cannot use this information anyway.