RFR: 7078: Update the p2-maven-plugin to 1.5.0

[Marcus Hirt]

Also cleaning up a bit.

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Commit messages:
 - 7078: Update the p2-maven-plugin to 1.5.0

Changes: https://git.openjdk.java.net/jmc/pull/188/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=188&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7078
  Stats: 23 lines in 1 file changed: 11 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/jmc/pull/188.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/188/head:pull/188

PR: https://git.openjdk.java.net/jmc/pull/188