A merge rebuilds what it changed, and what packages it

Three faults in one path. A merge rebuilt every module built from the repository, so one change in
a repository holding twenty-six of them meant twenty-six builds. A merge into a repository a module
only *packages* source from rebuilt nothing — two modules are built from the control plane's own
repository and neither had ever been rebuilt when it moved — because the manifest the mesh keeps
carries no build section, so a build now says which repositories it read and the mesh keeps that
beside what it stood on. And a module handed over by hand could record a repository with no
directory inside it, which is a module nothing can ever rebuild (novox/hq 04-ISSUES/131, /132).

A change inside no module's own directory is a change to what they share, and everything built from
that repository is rebuilt: rebuilding too much is the safe direction, because the fault this whole
path exists for is a mesh that believes it is current and is not.

cmd/mesh-controller/build.go

@@ -149,6 +149,9 @@ func buildFrom(result link.BuildResult) inventory.Build {
 	for _, ref := range result.Against {
 		kept.Against = append(kept.Against, catalogue.Recorded(ref))
 	}
+	for _, r := range result.Read {
+		kept.Read = append(kept.Read, inventory.ReadRepository{Repository: r.Repository, Ref: r.Ref})
+	}
 	var announced []inventory.Artifact
 	for _, made := range result.Made {
 		announced = append(announced, inventory.Artifact{