RFR: 7902878: JCov loses module hashes

[Leonid Kuskov]

This is the fix for https://bugs.openjdk.java.net/browse/CODETOOLS-7902878
JREInstr writes empty ModuleHashes attribute into java.base/module-info that cause 
a failure of the check of unavailability of qualified exports to upgradeable modules.
The fix removes ModuleHashes attribute from java.base/module-info which isn't needed 
for an instrumented JDK.(File: JREInstr.java)
Also the update contains minor code cleanup found during fixing.

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Commit messages:
 - 7902878: JCov loses module hashes

Changes: https://git.openjdk.java.net/jcov/pull/14/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jcov&pr=14&range=00
  Issue: https://bugs.openjdk.java.net/browse/CODETOOLS-7902878
  Stats: 281 lines in 8 files changed: 60 ins; 141 del; 80 mod
  Patch: https://git.openjdk.java.net/jcov/pull/14.diff
  Fetch: git fetch https://git.openjdk.java.net/jcov pull/14/head:pull/14

PR: https://git.openjdk.java.net/jcov/pull/14