RFR: 8254622: Hide superclasses from conditionally exported packages

[Hannes Wallnöfer]

Please review an enhancement to treat classes and interfaces that are not included and not unconditionally exported as hidden. This means they do not show up in the generated documentation even if they are implemented or extended by a documented type. 

This change makes the `@hidden` JavaDoc tag unnecessary in two internal base classes that previously used it, `jdk.internal.vm.vector.VectorSupport` and `jdk.internal.event.Event`. The generated documentation is unchanged.

The change also adds support for the `@hidden` JavaDoc tag in interfaces, which was previously missing, and adds coverage to `TestHiddenTag.java`.

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Commit messages:
 - Remove whitespace
 - Update copyright headers
 - Add test, support hidden interfaces
 - Merge branch 'master' into JDK-8254622
 - Tweak comments
 - 8254622: Hide superclasses from conditionally exported packages

Changes: https://git.openjdk.org/jdk/pull/24446/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24446&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8254622
  Stats: 382 lines in 19 files changed: 326 ins; 15 del; 41 mod
  Patch: https://git.openjdk.org/jdk/pull/24446.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24446/head:pull/24446

PR: https://git.openjdk.org/jdk/pull/24446