Leave a module out of a walk when its check built it the same as what runs (hq issue 721)

A one-sentence change in a package mesh-builder imports but never reaches moved
build-agent. A merge check now compares each module it builds with the build the
mesh holds, the whole image without file times, labels or creation time, and a
walk is cut without a module every check of its pull requests found the same,
while the mesh still holds that build. A comparison that fails moves the module
as before.

internal/link/build.go

@@ -238,6 +238,11 @@ type CheckBuilt struct {
 	// Kept names the check whose build this check kept instead of building, its key unchanged (novox/hq issue 703):
 	// a walk takes it as that check's build. Empty when this check built it.
 	Kept string `json:"kept,omitempty"`
+	// Same says every artifact came out the same as the build the mesh holds of the module, and SameAs the held
+	// reference each was compared with (novox/hq issue 721): a walk cut while the mesh still holds those builds leaves
+	// the module out. Empty when it differs, or could not be compared.
+	Same   string            `json:"same,omitempty"`
+	SameAs map[string]string `json:"same-as,omitempty"`
 }
 
 // GroupMember is one other head of a delivery group's composed check.