Allow to reference aggregator javadoc from plugin report

In some cases (for example in https://github.com/apache/maven-plugin-tools/ itself) the Javadoc is an aggregate being created by the aggregator module (after the plugin report is being executed). Currently those javadoc links are removed on parameter types, as they cannot be validated at the time when "report" is being executed on the individual modules (https://github.com/apache/maven-plugin-tools/blob/14e3d389e08626a52713ae4417e1f4056546a121/maven-plugin-tools-generators/src/main/java/org/apache/maven/tools/plugin/generator/PluginXdocGenerator.java#L569).