[MNG-8092] Incorrect warning about building plugins with extensions as part of reactor

**[Daniel Puiu](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=JIRAUSER285001)** opened **[MNG-8092](https://issues.apache.org/jira/browse/MNG-8092?redirect=false)** and commented

The warning on building plugin extension as part of reactor is logged even when the plugin is excluded from the reactor using --projects/-pl.

https://github.com/apache/maven/blob/master/maven-core/src/main/java/org/apache/maven/graph/DefaultGraphBuilder.java#L101 The method validateProjects is called on all collected projects before applying the filtering, so even when excluding the extension, the warn will still be printed.

I think the method (which only checks for this issue) could be easily renamed to validateExtensionIsNotPartOfReactor and be called on the filtered projects instead.

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**Affects:** 3.9.6