supportRelativeURL should handle source as a url

Despite the name of the argument, `supportRelativeURL`'s second argument, `url`, will be mangled if it is a fully-qualified URL, leading to error paths like `/Users/ckknight/Code/myproject/https:/github.com/ckknight/myproject/blob/...` instead of the expected URL.

This can occur when `"sources"` within a `.map` file contains references to URLs instead of relative paths.