Semantics: What should top-level runtime do if no projects use it?

[maxfierke]

Describe what you're trying to do:
When defining a top-level runtime configuration block, possibly with a default_version or a set of packages defined, what should mstrap do if no projects are found to use the runtime? Should default_version be installed? Should the defined packages be installed? If no default_version is set, should the latest version be installed?

Checklist:

• I checked the documentation site and the content is either not documented or the explanation was unclear (please expand, if so)