Flatten <repositories> part of deployed pom when using JEP-229

[Vlatombe]

Counterpart of jenkinsci/plugin-pom#802,
workarounds mojohaus/flatten-maven-plugin#362

Testing done

Updated pom of stapler with SNAPSHOT of this project

Ran

mvn -Pquick-build -Dset.changelist -Dignore.dirt verify

Verified that **/target/stapler-*-<version>.pom no longer has a <repositories> section.

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[basil]

Thanks for the PR!