A merge on the forge builds what it moved, bases first

The controller follows the forge's merges (novox/hq 04-ISSUES/131). For each
module recorded as built from that repository and branch it records the move to
the merge commit and builds it — bases first, because a module built before the
module it stands on is built against the old one and reports success, and a base
that fails stops what stands on it. Nothing is pushed here: what a finished build
does to the machines running the module stays the upgrade's decision.

Two more things the same ordering gives: `build --behind` builds bases first, and
`build --on <module>` rebuilds everything that stands on a module — the rebuild a
changed base needs, which "behind" does not see because their sources did not
move.

internal/link/receive_current.go

@@ -67,6 +67,10 @@ func Current(conn *amqp.Connection, channel *amqp.Channel) Inbound {
 // are events get their own queue each, and only when something is listening.
 func (c *currentInbound) Also(kind string) error {
 	switch kind {
+	case KindSourceMoved:
+		// Not followed on the bus the mesh is leaving: the forge's merges are announced on the
+		// new one, and this transport goes with the move (design 28, task 5.5).
+		return nil
 	case KindModuleMoved:
 		if err := c.bindEvent(UpgradeQueue, KeyModuleUpgraded); err != nil {
 			return err