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.

cmd/mesh-controller/plan_keys.go

@@ -25,19 +25,14 @@ import (
 // **No timer**: the builder's and the indexer's writes wake the look (builderkeys.Watch, watchNewModules).
 
 // isBuildersOwn says a module is build-agent's own: asked of build-agent on a merge and taken from its build record, never
-// from the builder's key, until R4g (ADR 0453 §5, ADR 0498 §7, ADR 0520). They are the builder's module, build-agent itself
-// (its image's context is another repository's) and the toolchain images build-agent's builds stand on
-// (builder.Bases): the builder writes no key of any of them, and a walk waiting for one would hold its tier for ever.
+// from the builder's key, until R4g (ADR 0453 §5, ADR 0498 §7, ADR 0520 §1).
 func isBuildersOwn(module string) bool { return buildersOwn(module) }
 
-// buildAgentsOwn are those modules: the builder's, build-agent's and every toolchain's base, read from the toolchains.
-var buildAgentsOwn = func() map[string]bool {
-	own := map[string]bool{sdkdelivery.BuilderModule: true, builder.AgentModule: true}
-	for _, b := range builder.Bases() {
-		own[b] = true
-	}
-	return own
-}()
+// buildAgentsOwn are those modules, named (ADR 0520 §1): the builder's; build-agent itself, whose image's context is another
+// repository's; and mesh-tools, the toolchain image build-agent's builds stand on. The builder writes no key of any of them,
+// and a walk waiting for one would hold its tier for ever. mesh-tools-go is not among them: it builds from a Dockerfile in
+// its own directory on a pinned base, and has its key from mesh-build-image.
+var buildAgentsOwn = map[string]bool{sdkdelivery.BuilderModule: true, builder.AgentModule: true, "mesh-tools": true}
 
 // buildersOwn is isBuildersOwn's rule: a variable only so a test of the asks left (their bus waits, their plan key) may
 // name modules of its own as build-agent's.