RFR: 8347334: JimageDiffGenerator code clean-ups [v4]

[Severin Gehwolf]

> During code review of [JDK-8346239](https://bugs.openjdk.org/browse/JDK-8346239) a few comments were made after the PR integrated. This follow-up patch cleans this up and adds a unit test for the `JimageDiffGenerator` class.
> 
> Testing:
> - [x] GHA
> - [x] tools/jlink tests (including the new one).
> 
> Thoughts?

Severin Gehwolf has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Merge branch 'master' into jdk-8347334-jimage-diff-gen-cleanup
 - Further simplification
 - Review feedback
 - 8347334: JimageDiffGenerator code clean-ups

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/23014/files
  - new: https://git.openjdk.org/jdk/pull/23014/files/c9cbb9a0..8b602b42

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23014&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23014&range=02-03

  Stats: 13212 lines in 322 files changed: 2494 ins; 9484 del; 1234 mod
  Patch: https://git.openjdk.org/jdk/pull/23014.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23014/head:pull/23014

PR: https://git.openjdk.org/jdk/pull/23014