.mvn/extensions.xml

<?xml version="1.0" encoding="UTF-8"?>
<extensions>
  <extension>
    <groupId>org.eclipse.tycho.extras</groupId>
    <artifactId>tycho-pomless</artifactId>
    <!-- must be same version as in ../pom.xml -->
    <version>1.7.0</version>
  </extension>
</extensions>