RFR: 7241: Adjust main pom.xml to more align for 3th party builds

[Patrick Reinhart]

Signed-off-by: Patrick Reinhart <patrick at reini.net>

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Commit messages:
 - 7241: Adjust main pom.xml to more align for 3th party builds

Changes: https://git.openjdk.java.net/jmc/pull/241/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=241&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7241
  Stats: 14 lines in 1 file changed: 4 ins; 10 del; 0 mod
  Patch: https://git.openjdk.java.net/jmc/pull/241.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/241/head:pull/241

PR: https://git.openjdk.java.net/jmc/pull/241