System order ambiguities should be grouped into related sets

## What problem does this solve or what need does it fill?

When multiple systems are ambiguous with each other, each pair is currently reported independently.

This is challenging to read and reason about, as the correct fix often involves thinking about the group as a whole.

## What solution would you like?

If a group of systems are all ambiguous with each other, report this as a single ambiguity.

## What alternative(s) have you considered?

We could instead group agglomeratively: if A is ambiguous with both B and C (but B and C are fine), group them.

This is probably incorrect: those errors tend to be due to several independent problems.

## Additional context

Please don't do this until #7267 is merged for the sake of my sanity.