RFR: 6982: CoreLibs: Add license and TPL info to sources and javadocs

[Guru Hb]

Adding "LICENSE.txt" and "THIRD_PARTY_LICENSES.txt" for "*-{source, javadoc}.jar" in core libraries.

Note: "maven-source-plugin" doesn't have capability to add resource outside the project directory. Added "org.codehaus.mojo:build-helper-maven-plugin" for copying license for the *-source.jar.

-------------

Commit messages:
 - 6982: CoreLibs: Add license and TPL info to sources and javadocs

Changes: https://git.openjdk.java.net/jmc/pull/164/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=164&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6982
  Stats: 29 lines in 1 file changed: 24 ins; 4 del; 1 mod
  Patch: https://git.openjdk.java.net/jmc/pull/164.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/164/head:pull/164

PR: https://git.openjdk.java.net/jmc/pull/164