Consider defining "URL path" as a concept

[annevk]

@johannhof and I have been looking at cookie layering, i.e., a refactoring of the Cookie RFC. One of the things that would be useful is to be able to refer to a URL path as its own concept, similar to how you can refer to a URL path segment today.

(I think we only need list-based URL paths, but it seems okay for the concept to encompass both. Perhaps other cookie clients want to support non-HTTP(S) schemes in some manner.)