modules list empty if modules don't use this project as parent in reactor build

The code in the AbstractSiteRenderingMojo does the following:

• if it's running in a reactor build (i.e. more than 1 project in the reactor projects) it scans all
  projects to see if it's parent project equals the current project. If so, it's marked as a module.

• if it's running on a single project, the project.build.modules is consulted and those modules
  are marked as modules.

I've got a 'fake' root pom, for utility purposes: it lists all projects as modules so that I can easily
built everything and generate a site. However, this fake root pom is never used as a parent - there's
a /pom/pom.xml project for that.

The result of this is that the modules list is empty.

A workaround is to first run 'mvn site' and then 'mvn site -N'.

I'm not sure what the correct solution should be - basically now 2 site layouts are implemented:

• a physical layout where the modules match the <modules> section of the pom, reflecting filesystem layout,
• a project hierarchy layout based on <parent>
  and one or the other is used depending on wheter the build contains more than 1 project or not.

My first feeling is that since the tag is called ${modules} (or <menu ref="modules"/>) that
the site should use the <modules>, not the <parent>.
It should also take into consideration, that IF a reactor build is running, it should only use the modules that are also
in the reactor (so you can use -r -Dmaven.reactor.excludes=.. to generate a site with not all projects in it).

Thoughts?