RFR: 8342642: Class loading failure due to archived map issue in ModuleLoaderMap.Mapper

[Jiangli Zhou]

Please review the fix that uses String type for the mapped value in ModuleLoaderMap.Mapper map (Map<String, String>). Please see details in https://bugs.openjdk.org/browse/JDK-8342642, thanks.

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Commit messages:
 - Use String instances as the loader indexes in ModuleLoaderMap.Mapper map.

Changes: https://git.openjdk.org/jdk/pull/21672/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21672&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342642
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/21672.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21672/head:pull/21672

PR: https://git.openjdk.org/jdk/pull/21672