Packer-plugin local development and unknown builder

I’m trying to work with the Developing Plugins documentation and the packer-plugin-scaffolding repo to create a builder.

Is it possible to reference this plugin locally? I’ve tried naming the compiled plugin packer-plugin-<xxx> and putting it in the .../.packer.d/pluings/ directory.

The init completes successful, and shows the plugin is found:

[TRACE] discovering plugins in /root/.packer.d/plugins
[DEBUG] Discovered plugin: testing = /root/.packer.d/plugins/packer-plugin-testing
[INFO] found external [testing-builder] builders from testing plugin

but the build returns a message as if the builder is not known.

[INFO] (telemetry) Finalizing.
  on  line 0:
  (source code not available)

known builders: [... hcloud vmware-vmx proxmox-clone
profitbricks oneandone file qemu yandex azure-arm jdcloud googlecompute triton
digitalocean null hyperv-iso tencentcloud-cvm cloudstack hyperv-vmcx
ucloud-uhost virtualbox-ovf lxd amazon-ebsvolume vagrant vsphere-iso ncloud
docker virtualbox-iso parallels-pvm amazon-chroot amazon-ebssurrogate azure-dtl
proxmox-iso alicloud-ecs virtualbox-vm amazon-ebs openstack amazon-instance
hyperone oracle-classic vmware-iso parallels-iso linode oracle-oci vsphere-clone
proxmox azure-chroot lxc]

Is there a way to make the builder “known” ? Should it be set to null ? or is something else going on?

with closer inspection, I see my builder is listed… but it had a name like testing-testing-builder … I’ll search the codebase, but maybe the question is still relevant. What’s required to get the builder picked up? … Seems some naming patterns don’t line up