RFR: 8240567: SystemModulesPlugin: Keep stack clean

[Oliver Kopp]

This refs [https://bugs.openjdk.org/browse/JDK-82405679](8240567). Does NOT fix it, but cleanup `SystemModulesPlugin` a bit to ease working on the bug itself.

Although this change is rather small, I think, it's good to have a "more clean" SystemModulesPlugin available.

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

Commit messages:
 - Keep stack clean

Changes: https://git.openjdk.org/jdk/pull/13442/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13442&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8240567
  Stats: 8 lines in 1 file changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13442.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13442/head:pull/13442

PR: https://git.openjdk.org/jdk/pull/13442