RFR: JDK-8309032: jpackage does not work for module projects unless --module-path is specified

[airsquared]

[JDK-8309032](https://bugs.openjdk.org/browse/JDK-8309032), [JDK-8306488](https://bugs.openjdk.org/browse/JDK-8306488)

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Commit messages:
 - Minor code cleanup in LauncherData.java
 - Fix ClassCastException in getPathListParameter: ImmutableCollections$List12 cannot be cast to class String

Changes: https://git.openjdk.org/jdk/pull/14840/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14840&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309032
  Stats: 16 lines in 1 file changed: 0 ins; 8 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/14840.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14840/head:pull/14840

PR: https://git.openjdk.org/jdk/pull/14840