RFR: 8536: Add LICENSE.txt and THIRD_PARTY_LICENSES.txt to the sources jars of jmc/core modules

[Aymane Harmaz]

This PR makes sure license files are included into sources jars of the jmc/core child modules.

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Commit messages:
 - 8536: Add LICENSE.txt and THIRD_PARTY_LICENSES.txt to the sources jars of jmc/core modules

Changes: https://git.openjdk.org/jmc/pull/708/files
  Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=708&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-8536
  Stats: 21 lines in 1 file changed: 19 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jmc/pull/708.diff
  Fetch: git fetch https://git.openjdk.org/jmc.git pull/708/head:pull/708

PR: https://git.openjdk.org/jmc/pull/708