Build a change once: the check builds what its merge moves, the walk takes it by key

A merge built every moved module a second time although its check had judged
the same tree (issue 649). The check now builds into the scratch namespace
under a reuse key (source fingerprint plus the builder's own digest), and the
walk of the trunk commit copies a build of the same key by digest instead of
building, saying per module why when it cannot (novox/hq ADR 0350).

internal/builder/source.go

@@ -64,6 +64,43 @@ type sourceInputs struct {
 	// contextsAtHead is false once a context was cloned at something other than its trunk's head: its
 	// closure is not the one the next merge there meets, and the build says no build source.
 	contextsAtHead bool
+	// onlyKey says the inputs are being taken for a reuse key (novox/hq ADR 0350 §2): each artifact goes as far
+	// as what its source fingerprint reads, and nothing is built or published.
+	onlyKey bool
+}
+
+// keyOnly is whether these inputs are only being taken for a reuse key.
+func (s *sourceInputs) keyOnly() bool { return s != nil && s.onlyKey }
+
+// copy is the inputs taken so far, apart: a walk's key is taken from them without touching the build's own.
+func (s *sourceInputs) copy() *sourceInputs {
+	c := newSourceInputs(s.module)
+	c.tree, c.unpinned, c.prefix, c.ownIsAll, c.contextsAtHead = s.tree, s.unpinned, s.prefix, s.ownIsAll, s.contextsAtHead
+	c.bases = append([]string{}, s.bases...)
+	for k, v := range s.contexts {
+		c.contexts[k] = v
+	}
+	for k, v := range s.toolchains {
+		c.toolchains[k] = v
+	}
+	for k, v := range s.own {
+		c.own[k] = v
+	}
+	for k, v := range s.read {
+		if v == nil {
+			c.read[k] = nil
+			continue
+		}
+		set := map[string]bool{}
+		for e := range v {
+			set[e] = true
+		}
+		c.read[k] = set
+	}
+	for k, v := range s.readAs {
+		c.readAs[k] = v
+	}
+	return c
 }
 
 // contextOffHead says a context was not its trunk's head.