Say why each module is in a plan: its build source's changed files, or why it is read whole (hq ADR 0267, issue 363)

A what-if named the build seat's holder as packaging the controller's
source, "rebuilt without their own source moving", while it was in the
plan because an open plan had not built it yet. The what-if and the merge
log now say, per module, which changed files of its build source moved it,
or that it is read whole and why: no build source recorded, a newer build
failed, or a plan has not built it yet.

internal/inventory/builds.go

@@ -98,6 +98,9 @@ type ReadRepository struct {
 	// LookedAt are the merge commits a plan that built the module, or is building it, answered: a merge
 	// of one of them is history for the module whatever the clocks say. Never stored.
 	LookedAt []string `json:"-"`
+	// Whole is why the module is read whole though a build source may have been recorded — a newer build
+	// failed, or a plan has not built it yet — said on an Own entry with no Paths. Never stored.
+	Whole string `json:"-"`
 }
 
 // BuildSource is the build source a build read in one repository (novox/hq ADR 0267): Repository and Ref
@@ -408,10 +411,15 @@ func (i *Inventory) ReadRepositories(ctx context.Context) (map[string][]ReadRepo
 				sources = nil
 			}
 		}
-		if n, known := newest[module]; known && n.failed && n.at.After(built) {
-			sources = nil
+		whole := ""
+		if n, known := newest[module]; known && n.failed && n.at.After(built) && len(sources) > 0 {
+			sources, whole = nil, "a newer build of it failed"
 		}
-		if of = WithBuildSources(of, sources); len(of) > 0 {
+		of = WithBuildSources(of, sources)
+		if whole != "" {
+			of = append(of, ReadRepository{Own: true, Whole: whole})
+		}
+		if len(of) > 0 {
 			for k := range of {
 				of[k].Built, of[k].Looked = built, newest[module].at
 			}

internal/inventory/builds_test.go

@@ -243,7 +243,9 @@ func TestAFailedNewerBuildLeavesTheBuildSourceStale(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if got := read["route-proxy"]; len(got) != 1 || got[0].Paths != nil || got[0].Own || !got[0].Looked.After(got[0].Built) {
+	got := read["route-proxy"]
+	if len(got) != 2 || got[0].Paths != nil || got[0].Own || !got[0].Looked.After(got[0].Built) ||
+		!got[1].Own || got[1].Paths != nil || got[1].Whole != "a newer build of it failed" {
 		t.Fatalf("after a newer failed build the proxy reads as %+v", got)
 	}
 }