[MNG-7222] Validate and warn for naming collisions between Maven and System properties

**[Konrad Windszus](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=kwin)** opened **[MNG-7222](https://issues.apache.org/jira/browse/MNG-7222?redirect=false)** and commented

As properties referenced via `${x`} are first being resolved against system properties and only afterwards against Maven properties according to https://maven.apache.org/pom.html#properties. As this is often not noticed such collisions should be detected by Maven and lead to a WARN.

An example for such a collision can be seen at https://github.com/adobe/aem-core-wcm-components/blob/6a294b0483c711a0283474f962a4643234e47c6e/parent/pom.xml#L64 which is never effective as this is always overridden by https://docs.oracle.com/javase/tutorial/essential/environment/sysprop.html

At least the mandatory properties (available on all JREs) outlined at https://docs.oracle.com/javase/8/docs/api/java/lang/System.html#getProperties-- should always lead to a WARN.

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