Fail the build on wrong import order

PR: https://github.com/xwiki/xwiki-commons/pull/1908

It must be merged only after the 3 sweeps fixing the existing violations, since the Checkstyle configuration is shared by the 3 repositories:

• xwiki-commons: https://github.com/xwiki/xwiki-commons/pull/1905
• xwiki-rendering: https://github.com/xwiki/xwiki-rendering/pull/406
• xwiki-platform: https://github.com/xwiki/xwiki-platform/pull/6191

Waiting for agreement on https://forum.xwiki.org/t/fail-the-build-on-wrong-import-order/18733 before applying.