RFR: 8318812: LauncherHelper.checkAndLoadMain closes jar file that's about to be re-opened

[Christian Stein]

Please review this PR that makes the launcher helper keep a reference to the executable JAR file active after extracting the name of the main class and returning it as Class instance. Now, when loading classes from the JAR file, it hasn't to be re-opened.

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Commit messages:
 - Fail fast
 - Simplify implementation
 - 8318812: LauncherHelper.checkAndLoadMain closes jar file that's about to be re-opened

Changes: https://git.openjdk.org/jdk/pull/17843/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17843&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318812
  Stats: 18 lines in 1 file changed: 15 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17843.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17843/head:pull/17843

PR: https://git.openjdk.org/jdk/pull/17843