Component start page should be constrained to be within the component version.

Specifying a start_page in an antora.yml component descriptor should be constrained to be in that component and version. At the moment, you can specify any component and version, thus making the component inaccessible except through direct links into pages. For instance, this works:

name: component4 
title: Component 4 
version: '1.0' 
start_page: component3::other.adoc 
nav: 
- modules/ROOT/nav.adoc