RFR: 8311591: Add SystemModulesPlugin test case that splits module descriptors with new local variables defined by DedupSetBuilder [v6]

[Christoph]

> Add new test case with sample modules that contains some requires/exports/uses/provides.
> 
> We are just unsure if and how we should add some last step of verificaiton with the extracted and decompiled class.
> 
> Follow up task from https://github.com/openjdk/jdk/pull/14408

Christoph has updated the pull request incrementally with one additional commit since the last revision:

  Fix missing import, use size instead of count

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Changes:
  - all: https://git.openjdk.org/jdk/pull/15234/files
  - new: https://git.openjdk.org/jdk/pull/15234/files/015a3c2e..2ebc0592

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15234&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15234&range=04-05

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15234.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15234/head:pull/15234

PR: https://git.openjdk.org/jdk/pull/15234