Handle `wasi:cli/run` specially for a command component

[alexcrichton]

Right now if a command component is created and a world is included via:

package example:demo 

world demo {
    include wasi:cli/command
}

then the output component will expect a Component which implements the wasi:cli/run interface. This is already implicitly created for the "command" flavor of the component and the adapter, though, so could this world be modified in this situation implicitly by cargo component to avoid the requirment that impl Guest for Component is needed to compile?