refactor(ish): reduce number of topology lists

Having 6 lists of topolgies in the same file is a bit much:

1. consts for topology names
2. Report type definition
3. MakeReport() Report initialisation
4. Report.Topology(name) lookup
5. Report.TopologyMap() mapping of names to topology references
6. Report.WalkPairedTopologies() iterator over topology references

We get rid of 5 and 6 by introducing a topologyNames slice. So we
are down to 5.

We replace Report.TopologyMap() with a new function,
WalkNamedTopologies, that uses topologyNames. WalkPairedTopologies()
is updated to operate in a similar fashion. Likewise for
WalkTopologies() and Topologies() - these were previously calling
Walk[Paired]Topologies, but it is clearer to simply implement them
directly.

probe/topology_tagger.go

@@ -16,10 +16,10 @@ func (topologyTagger) Name() string { return "Topology" }
 
 // Tag implements Tagger
 func (topologyTagger) Tag(r report.Report) (report.Report, error) {
-	for name, t := range r.TopologyMap() {
+	r.WalkNamedTopologies(func(name string, t *report.Topology) {
 		for _, node := range t.Nodes {
 			t.AddNode(node.WithTopology(name))
 		}
-	}
+	})
 	return r, nil
 }