It appears that on all recent versions of macOS when adding a new InputSource in <domain>/Library/Input Methods (or modifying an existing one there) the user needs to logoff and log back in in order for Keyboard/Input Sources in System Settings and Input Menu in menu bar to pick up the changes.
Is there a way to avoid this? That is, some notification to send or API to call to tell both of these "hey, things might have changed on disk, please re-read the info, and update the UI". 🙂