[regression] User properties override equally named POM properties of transitive dependencies

While trying to run the core ITs using -DmavenVersion=3.0.1 the build ended up with a resolution failures for a bunch of non-existing artifacts like maven-reporting-api:3.0.1. The problematic artifact versions were coming from a wrong dependency tree like this:

DEBUG] org.apache.maven.plugins:maven-resources-plugin:jar:2.3:
DEBUG]    org.apache.maven:maven-plugin-api:jar:2.0.6:compile
DEBUG]    org.apache.maven:maven-project:jar:2.0.6:compile
DEBUG]       org.apache.maven:maven-settings:jar:3.0.1:compile
DEBUG]       org.apache.maven:maven-profile:jar:3.0.1:compile
DEBUG]       org.apache.maven:maven-model:jar:3.0.1:compile
DEBUG]       org.apache.maven:maven-artifact-manager:jar:3.0.1:compile
DEBUG]       org.apache.maven:maven-plugin-registry:jar:3.0.1:compile
DEBUG]       org.apache.maven:maven-artifact:jar:3.0.1:compile

Note that the dependencies of maven-project:2.0.6 use the wrong version.

The problem is due to maven:pom:2.0.6 using the POM property mavenVersion which erroneously gets overridden by my user property.

During dependency resolution, user properties must be treated like system properties and must not override POM properties.