RFR: 8252173: Use handles instead of jobjects in modules.cpp

[Harold Seigel]

Hi,
Please review this change for JDK-8252173 to use handles instead of jobjects in modules.cpp to make modules.cpp more debuggable.  The change was tested with Mach5 tiers 1 and 2 on Linux, Mac OS, and Windows, and tiers 3-5 on Linux x64.

Thanks, Harold

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Commit messages:
 - 8252173: Use handles instead of jobjects in modules.cpp

Changes: https://git.openjdk.java.net/jdk/pull/2878/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2878&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252173
  Stats: 74 lines in 4 files changed: 17 ins; 5 del; 52 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2878.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2878/head:pull/2878

PR: https://git.openjdk.java.net/jdk/pull/2878