supportRelativeURL should handle source as a url

[ckknight]

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.

[pfhayes]

Proposed fix is here: #332