[Vuejs]-How to make declaration files for an unofficial vue module, or any packages for that matter?

Another workaround to the one you suggest is to create a vue-clickaway.d.ts file with the following content:

declare module 'vue-clickaway' {
  export const mixin: any;
}

This is still a workaround as this doesn’t really add typings to the module, but only makes any explicit.