fixes #14513. (I also added a drive-by fix for the fact that non-relative resolution ignored paths.base.)

In the process of doing this, I realised that resolve(...) will give incorrect results when called in the context of a remote function, since the pathname is always / in that case. Not entirely sure how best to handle it — we should probably fall back to non-relative resolution in that case too? (Making it relative to the route from which the remote function was called isn't a solution, because then the result is non-deterministic and non-cacheable.)

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