libtopo should probably be "not optional" to view disks #11319

History lesson: when we wrote sled agent scanning for disks, we used libdevinfo, monitoring device drivers, as a proxy for "what disks exist". This sorta works, but has the downside that "disks which are physically attached, but have problematic drivers" may not actually be visible.

Now that we're integrating libtopo (#11279) it seems worthwhile re-visiting this disposition. #11279 took the additive incremental approach of "add libtopo as an optional extra step, to access label information", but this integration should, if possible, be "non-optional".

Ideally, we should:

• Scan libtopo to get a view of hardware occupancy
• Scan libdevinfo to get a view of drivers
• Synchronize the two, and treat "abscence from libtopo" as a non-nullable pathway