RFR: 8335870: Doclet should default @since of classes with no @since information to that of the enclosing package

[Nizar Benalla]

Please review this change to default the value of `@since` to that of the immediately enclosing package if a class doesn't have the `@since` tag.

For nested classes, you still need to recursively look for the package so I had to add an extra check.

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Commit messages:
 - Extend createWithDefaultForNested to look for @since in the package

Changes: https://git.openjdk.org/jdk/pull/20918/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20918&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335870
  Stats: 61 lines in 2 files changed: 58 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20918.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20918/head:pull/20918

PR: https://git.openjdk.org/jdk/pull/20918