When the build system tries to figure out which modules that should be included by the javadoc build, it locates the set of modules "required" by already included modules, starting from an initial set and repeating recursively (a method which we unfortunately called "transitive").

However, for javadoc, we should only look at those re-exported modules that are classified by a "requires transitive" relationship (which I therefore have called "re-exported" to not clash with the established term "transitive").

The effect of getting this wrong is e.g. that we copy module graph png files that we should not, and that we look at too broad a set of files for looking at changes that should trigger a re-build of the javadoc.