[MNG-5102] Add support for POM mixins

Author: gnodet

api/maven-api-model/src/main/mdo/maven.mdo

@@ -113,6 +113,20 @@
           </association>
         </field>
 
+        <!-- ====================================================================== -->
+        <!-- Mixins                                                                 -->
+        <!-- ====================================================================== -->
+
+        <field xdoc.separator="blank">
+          <name>mixins</name>
+          <version>4.1.0+</version>
+          <description>Mixins...</description>
+          <association>
+            <type>Parent</type>
+            <multiplicity>*</multiplicity>
+          </association>
+        </field>
+
         <!-- ====================================================================== -->
         <!-- groupId/artifactId/Version/Packaging                                   -->
         <!-- ====================================================================== -->

maven-core/src/main/java/org/apache/maven/internal/transformation/ConsumerPomArtifactTransformer.java

@@ -236,6 +236,7 @@ static void transform(Model model, Path dest) {
                             .downgradeModelVersion(false)
                             .root(false)
                             .parent(null)
+                            .mixins(null)
                             .build(null),
                     model);
             if (isBom) {

maven-core/src/main/java/org/apache/maven/model/plugin/DefaultLifecycleBindingsInjector.java

@@ -97,10 +97,10 @@ public Model merge(Model target, Model source) {
                 targetBuild = Build.newInstance();
             }
 
-            Map<Object, Object> context = Collections.singletonMap(
-                    PLUGIN_MANAGEMENT, target.getBuild().getPluginManagement());
+            Map<Object, Object> context =
+                    Collections.singletonMap(PLUGIN_MANAGEMENT, targetBuild.getPluginManagement());
 
-            Build.Builder builder = Build.newBuilder(target.getBuild());
+            Build.Builder builder = Build.newBuilder(targetBuild);
             mergePluginContainer_Plugins(builder, targetBuild, source.getBuild(), false, context);
 
             return target.withBuild(builder.build());