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.

Locate the supportRelativeURL entry point and inspect how its second argument is handled when a source-map "sources" entry is a fully-qualified URL. Reproduce the malformed path and verify that URL sources remain intact while relative paths continue to work.

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