Improve Javadoc task to document `requires` entries

[tlinkowski]

Currently, javadoc task generates:

• overview (from a JavaDoc comment on module-info.java)
• "Packages" section (from exports in module-info.java)

Example:

• module-info.java
• generated module JavaDoc

However, javadoc doesn't generate:

• "Indirect Exports" subsection (from exports of transitively required modules)
• "Modules" section (from requires in module-info.java)

Example:

• java.se JavaDoc with "Indirect Exports" and "Modules" section

---

I tried to figure out what needs to be done to achieve this effect, but no success so far.

I quickly tried the following options:

• --module: got Module ___ not found message
• --module-source-path: had trouble to get it working
• -Xmodule:module-name: got a message that this options is not supported

[tkslaw]

The closest I could find for this is --expand-requires=transitive (documented here) but that seems to be meant for documenting multiple modules together. For instance, when I tried that option while my module had requires transitive java.xml; I received two warnings saying the "module xml could not be found"; the documentation still generated and java.xml showed up, but the links were completely messed up (even with -link). However, when I created two modules where one transitively required the other (with no other transitive dependencies) everything seemed to work.