[MPOM-337] Load plugins containing lifecycle mapping/artifact handler with <extensions>true</extensions>

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MPOM-337](https://issues.apache.org/jira/browse/MPOM-337?redirect=false)** and commented

As preparation of MNG-5697 all plugins containing a custom default lifecycle mapping and/or artifact handler should be loaded with `true`.

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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
(_**"is caused by"**_)
- [MNG-7574](https://issues.apache.org/jira/browse/MNG-7574) Never expose Mojo classes via Core Classloader

**Remote Links:**
- [GitHub Pull Request #92
](https://github.com/apache/maven-apache-parent/pull/92)

Start by reviewing MPOM-337 and the linked MNG-5697 and MNG-7574 issues to understand the lifecycle-mapping and artifact-handler changes. Locate the plugin-loading configuration and verify that plugins with custom default lifecycle mappings or artifact handlers are loaded with true; check the related remote pull request for existing work.

Written by the indexing model from the issue text.