Name build-agent's own as the builder's module, build-agent and mesh-tools (ADR 0520 §1, issue 1851)

ADR 0520 narrowed after its review: mesh-tools-go builds from its own
directory on a pinned base and has its key from mesh-build-image, and
mesh-tools-python is no module, so the set is named, not derived from
the toolchain bases.

internal/builder/toolchain.go

@@ -237,21 +237,6 @@ func spoken() string {
 // 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 {