Find site start page in component version promoted to site root

If a component version is promoted to the site root, ContentCatalog#getSiteStartPage should recognize the start page for that component version as the site start page. This happens when the component name is ROOT and the latest version or prerelease version segment is empty. In this case, the pub.url for the start page for that component version is equivalent to the root index page. Antora should recognize this as an implicit site start page.

Note that Antora already recognizes the implicit start page when the version of the component version is empty string. This issue deals with the case when the version is non-empty (and is masked by the latest version or latest prerelease version segment).