Say a build source only for the trunk's head, and hold what C, assembly and a new go.mod reach (hq ADR 0267, review)

A hand build of an older trunk commit said a closure lacking what was
imported since, and the planner would have mapped the next merge onto it.
C and assembly beside Go may include files below their directory, and a
go.mod made above a package moves it out of its module: each is now held.

internal/builder/source.go

@@ -61,11 +61,22 @@ type sourceInputs struct {
 	// read is, per context (repository and ref), the build source read there; nil for one read whole.
 	read   map[string]map[string]bool
 	readAs map[string]catalogue.ArtifactContext
+	// 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
+}
+
+// contextOffHead says a context was not its trunk's head.
+func (s *sourceInputs) contextOffHead() {
+	if s != nil {
+		s.contextsAtHead = false
+	}
 }
 
 func newSourceInputs(module string) *sourceInputs {
 	return &sourceInputs{module: module, contexts: map[string]string{}, toolchains: map[string]string{},
-		own: map[string]bool{}, read: map[string]map[string]bool{}, readAs: map[string]catalogue.ArtifactContext{}}
+		own: map[string]bool{}, read: map[string]map[string]bool{}, readAs: map[string]catalogue.ArtifactContext{},
+		contextsAtHead: true}
 }
 
 // ownHas adds entries, relative to the module's directory, to its build source in its own repository.
@@ -156,6 +167,15 @@ func (s *sourceInputs) buildSources() []BuildSource {
 
 // withPrefix is an entry relative to the module's directory made relative to its repository's root.
 func withPrefix(prefix, entry string) string {
+	switch {
+	case entry == "." || entry == "./":
+		entry = ""
+	case entry == "**" || entry == "./**" || entry == "/**":
+		if prefix == "" {
+			return "**"
+		}
+		return prefix + "/**"
+	}
 	entry = strings.TrimPrefix(entry, "./")
 	if prefix == "" {
 		if entry == "" {