RFR: 8277165: jdeps --multi-release --print-module-deps fails if module-info.class in different versioned directories

[Mandy Chung]

jdeps intends to report an error if there are multiple versions of the same class being parsed.   module-info.class should be excluded from such detection.

This patch also fixes a data race in `VersionHelper::set` and also unwraps the `ExecutionException` when FutureTask of parsing the classes throws an exception to report `MultiReleaseException` properly.

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Commit messages:
 - add comment to the test
 - JDK-8277165: jdeps cannot handle 2 jars that put module-info.class in a different versioned directory

Changes: https://git.openjdk.java.net/jdk/pull/6530/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6530&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277165
  Stats: 301 lines in 11 files changed: 288 ins; 6 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6530.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6530/head:pull/6530

PR: https://git.openjdk.java.net/jdk/pull/6530