A build's outcome belongs to the role, not to the module holding it

ADR 0121. The builder declared `built` as its own event, so every consumer depended
on which module happens to be the build machine today. It is the build-machine
role's event now: the builder declares none of its own, and the catalogue listens
for `mesh-build-machine.built` rather than `builder.built`.

Nothing changes about what reaches the catalogue. What changes is that it survives
the build machine being a different module, which is the whole reason the mesh has a
word for a role.

modules/builder/module.json

@@ -20,9 +20,6 @@
   "secrets": {
     "npm-package-registry": "/var/lib/mesh/builder/package-registry.secret"
   },
-  "emits": [
-    "built"
-  ],
   "own-secrets": {
     "broker": "/var/lib/mesh/builder/broker"
   },