RFR: 8318082: ConcurrentModificationException from IndexWriter

[Pavel Rappo]

Please review this fix to a bug in indexing.

The bug has been dormant for many releases and was noticed only because its likelihood was increased by [JDK-8286470](https://bugs.openjdk.org/browse/JDK-8286470) in JDK 21.

Here's the bug: every time an item is indexed, it is indexed relative to the context that triggers the indexing. For example, if an item is indexed in the context of a class page, then the index will refer to that class page. However, if that same item is indexed in the context of the "Deprecated" page, then the index will refer to the "Deprecated" page.

Since an item is indexed every time it is seen, the fix is to never index an item if seen in the context of a _derived_ page, such as hierarchy trees, indexes, and various lists (e.g. deprecated, new, preview). The fix includes a comprehensive test and updates an existing test to correct its bad assumptions.

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Commit messages:
 - Please jcheck
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/16271/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16271&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318082
  Stats: 205 lines in 4 files changed: 172 ins; 28 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/16271.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16271/head:pull/16271

PR: https://git.openjdk.org/jdk/pull/16271