RFR: JDK-8254627: Cleanup {Abstract,Single,Split}IndexWriter classes

[Jonathan Gibbons]

Inspired by other recent work to cleanup index generation, this change is primarily about cleanup for
`AbstractIndexWriter`, `SingleIndexWriter`, `SplitIndexWriter` to reduce code duplication and to reduce the 3 class to
just 1 that can handle either single index files or split index files.

As part of this cleanup, some anomalies were uncovered in the naming of annotation types, enum constants and records,
which have also been addressed.

The changes involved moving/merging code from `SingleIndexWriter` and `SplitIndexWriter` in to `AbstractIndexWriter`,
which is now no longer abstract and is thus renamed to just `IndexWriter`. Regrettably, the rename is not properly
tracked.

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Commit messages:
 - JDK-8254627: Cleanup {Abstract,Single,Split}IndexWriter classes

Changes: https://git.openjdk.java.net/jdk/pull/621/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=621&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254627
  Stats: 1066 lines in 13 files changed: 420 ins; 607 del; 39 mod
  Patch: https://git.openjdk.java.net/jdk/pull/621.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/621/head:pull/621

PR: https://git.openjdk.java.net/jdk/pull/621