RFR: JDK-8175824 Adapt javadoc generation to different requirements for JDK and JavaSE

[Magnus Ihse Bursie]

javadoc allows a file to be specified to provide content for the top 
level "overview" page.

As we move towards a unified docs bundle, we need to be able to vary the 
content of that file depending on the content of the bundle. This does 
not mean providing or including lists of modules, but should reflect 
whether the bundle contains Java SE modules, JDK modules and JavaFX 
modules.

This patch also includes support for (and relies on) the new usage of 
the -group option from JDK-8180336.

Note that when JDK-8180480 (Use "requires transitive" relationship when 
determining modules for javadoc) is pushed, the call to 
FindTransitiveDepsForModules for JavaSE_MODULES should be updated to the 
new FindTransitiveIndirectExportsForModules (or whatever it'll end up 
being called). I'll fix that in whichever of the two bugs I push last.

This patch also contains some accumulated cleanup in Docs.gmk after all 
the piecemeal patches this file has recieved for the last few weeks.

Note: While I'm willing to make minor changes to the actual contents of 
the generated overview.html, I'd prefer if any larger (or potentially 
controversial) issues are handled as separate follow-up bugs. With this 
framework in place, it's easy to modify whatever gets written to the file.

An example how this looks like for the JDK and Java SE documentation 
collections (stripped down to the summary page of the javadoc output, 
and the JDK top-level index.html) is here:
http://cr.openjdk.java.net/~ihse/demo-generated-overview/

Bug: https://bugs.openjdk.java.net/browse/JDK-8175824
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8175824-javadoc-dynamic-overview-and-groups/webrev.01