RFR: JDK-8180480 Use "requires transitive" relationship when determining modules for javadoc

[Magnus Ihse Bursie]

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.

Bug: https://bugs.openjdk.java.net/browse/JDK-8180480
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8180480-docs-should-use-requires-transitive/webrev.01

/Magnus