[MJAVADOC-727] Make methods from JavadocUtil public which deal with retrieving "package-list"/"element-list"

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MJAVADOC-727](https://issues.apache.org/jira/browse/MJAVADOC-727?redirect=false)** and commented

The functionality added in MPLUGIN-417 requires linking to javadoc class pages. In order to distinguish multiple javadoc site sources it needs access to the following methods:
1. https://github.com/apache/maven-javadoc-plugin/blob/231316be785782b61d96783fad111325868cfa1f/src/main/java/org/apache/maven/plugins/javadoc/JavadocUtil.java#L1694
2. https://github.com/apache/maven-javadoc-plugin/blob/231316be785782b61d96783fad111325868cfa1f/src/main/java/org/apache/maven/plugins/javadoc/JavadocUtil.java#L1818

which help evaluating the `package-list / element-list` provided by a javadoc generated site.

---

**Issue Links:**
- [MPLUGIN-417](https://issues.apache.org/jira/browse/MPLUGIN-417) report and descriptor goal need to evaluate Javadoc comments differently

Start in src/main/java/org/apache/maven/plugins/javadoc/JavadocUtil.java at the two linked locations. Inspect the methods that retrieve package-list and element-list data, then make those methods publicly accessible for the Javadoc site-source evaluation described in the issue. Done means callers can access both methods without changing their retrieval behavior.

Written by the indexing model from the issue text.