Fix aggregated multi-module Javadoc

[sjoerdtalsma]

Expected Behavior

Aggregated Javadoc generated from the parent maven module.

Actual Behavior

• Breaking Javadoc build (see Bump maven-javadoc-plugin from 3.0.1 to 3.2.0 #157).
• Manually having to maintain package references in https://github.com/talsma-ict/context-propagation/tree/develop/.mvn/javadoc which is a maintenance burden
• We should have at least an aggregated package-dependencies UML diagram at the root javadoc.