* Move `file`, `ssh` and `user` packages to internal
* Rename `SSHKey` to `(ssh.)UserKey`
* Rename KeyValue / Publickey to PublicKey
* Rename SSH key file from "debugd" to "ssh-keys"
* Add CreateSSHUsers function to Core
* Call CreateSSHUsers users on first control-plane node, when defined in config
Tests:
* Make StubUserCreator add entries to /etc/passwd
* Add NewLinuxUserManagerFake for unit tests
* Add unit tests & adjust existing ones to changes
