Ask the planner what a pull request reaches; map a changed file onto modules in one place (hq ADR 0238)

touchedBy is now the only mapping of changed files onto modules — touched, added, and
read by no build — and reachOfMerge the planner's whole answer with the dependency walk.
The merge handler, the plan what-if, the merge gate's width and composition, and a pull
request's check all ask it, so planning and gating cannot disagree. The gate composes
the definitions of the modules a merge would rebuild or add, not every one in the tree,
and the check says the dependents a merge would build after them.

cmd/mesh-controller/release_plan.go

@@ -1357,18 +1357,20 @@ func planWhatIf(ctx context.Context, inv *inventory.Inventory, repository string
 	for _, name := range modules {
 		named[name] = true
 	}
-	for _, e := range entries {
-		switch {
-		case named[e.Manifest.Module]:
-			from = append(from, e)
-		case len(named) == 0 && sourceIs(e.Source, m):
-			from = append(from, e)
-		case readsFrom(read[e.Manifest.Module], m):
-			packaging = append(packaging, e)
-		}
-	}
 	if len(named) == 0 {
-		from = whatTheMergeTouched(from, entries, m)
+		// The changed files: the planner's own answer, as the merge handler, the merge gate and a pull
+		// request's check ask it (novox/hq ADR 0238).
+		r := reachOfMerge(m, entries, read, nil)
+		from, packaging = append(append([]inventory.Entry{}, r.Touched...), r.Deleted...), r.Packaging
+	} else {
+		for _, e := range entries {
+			switch {
+			case named[e.Manifest.Module]:
+				from = append(from, e)
+			case readsFrom(read[e.Manifest.Module], m):
+				packaging = append(packaging, e)
+			}
+		}
 	}
 	moved := append(append([]inventory.Entry{}, from...), packaging...)
 	if len(moved) == 0 {