[MNG-7508] Validate usage of <extensions>true</extensions> on plugins

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MNG-7508](https://issues.apache.org/jira/browse/MNG-7508?redirect=false)** and commented

It should be validated by Maven if a Maven plugin loaded with `true` actually comes with extensions or custom lifecycle mapping requiring it to be loaded with that option. Similarly in case this flag is `false` (default) Maven should emit a warning if the plugin comes with extensions.

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**Issue Links:**
- [MNG-5697](https://issues.apache.org/jira/browse/MNG-5697) remove LifecycleMapping and ArtifactHandler from maven-core: move to target packaging plugin

- [MPLUGIN-402](https://issues.apache.org/jira/browse/MPLUGIN-402) report: allow to generate usage section in plugin-info.html with \true\