Add option for rootfs inventory data

ScanCode.io currently provides Resource data for each layer. This is what we need for most SCA scenarios but it is cumbersome when you want to see the "union" root filesystem content in the RESOURCES worksheet regardless of how it was created. The primary use case is when you want to evaluate the content of an image top-down. This would be an optional alternative view of the image Resources, not a replacement for the current RESOURCES data or output worksheet.