Require some vendor package
Use some composer or some version control system link. This feature is like jsfiddle.net External Resources
With this you can test some feature of vendor package.
AdminSjon Hortensius (Admin, 3v4l) commented
Is there a list of most-popular Composer packages or should this be completely custom and allow specifying any package & version from packagist.org ?
Asked a few months ago but I totally agree: a simple list of Composer packages would be a really great first step! We could even test packages development phase and many cool stuff :)