Ask build-agent for itself and its toolchain images, never a builder key (ADR 0520, issue 1851)

The builder writes no key of build-agent or of the toolchain bases its
builds stand on, so a walk moving one would hold its tier for ever.
Until R4g they are build-agent's own, as the builder's module is; the
bases are read from the toolchains, not copied.

internal/builder/toolchain.go

@@ -234,6 +234,24 @@ func spoken() string {
 }
 
 // Languages is every language the mesh can build, for anything that wants to say so.
+// AgentModule is build-agent's own module: the machine's builder, whose image is built from another repository's context.
+const AgentModule = "build-agent"
+
+// Bases are the modules whose artifacts provide the toolchains' compilers (Toolchain.Base), each once, sorted: images
+// build-agent's builds stand on.
+func Bases() []string {
+	seen := map[string]bool{}
+	var out []string
+	for _, t := range toolchains {
+		if t.Base != "" && !seen[t.Base] {
+			seen[t.Base] = true
+			out = append(out, t.Base)
+		}
+	}
+	sort.Strings(out)
+	return out
+}
+
 func Languages() []string {
 	names := make([]string, 0, len(toolchains))
 	for _, t := range toolchains {