RFR: 8372055: Fragment link tag does not work in package info

[Hannes Wallnöfer]

Please review a change to make unqualified anchor references such as `{@link ##anchor link}` work everywhere. Previously, such links only worked in class pages. The fix consists in attributing the correct Element in `JavacTrees::attributeDocReference` for references declared in API elements, and setting it to `null` for references defined in `doc-files` to make them work as local links.

Because of unattributed `doc-files` references, we also have to exclude such links from the reference check in DocLint. This is fine because local references always have a target element, and we do not check the anchor anyway.

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Commit messages:
 - Make unqualified reference in legacy package.html work as package link everywhere.
 - Add tests and tweak details
 - 8372055: Fragment link tag does not work in package info

Changes: https://git.openjdk.org/jdk/pull/28372/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28372&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372055
  Stats: 156 lines in 4 files changed: 135 ins; 0 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/28372.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28372/head:pull/28372

PR: https://git.openjdk.org/jdk/pull/28372