`resolve(...)` works incorrectly inside a remote function

[Rich-Harris]

Describe the bug

In a remote function, event.url.pathname is always / (because the actual pathname of the generated endpoint is an implementation detail that shouldn't be relied upon).

That means that resolve(...) will always return a pathname that's resolved from the root, which generally isn't what people intend, since they can't then use the pathname for anything useful.

We could fall back to non-relative resolution in this case, but that would cause breakage for people who rely on relative pathnames. Or we could just forbid its use altogether. Not sure.