[MNG-7634] Revert MNG-5982 and MNG-7417

This closes #916

Author: michael-o

maven-resolver-provider/src/main/java/org/apache/maven/repository/internal/DefaultArtifactDescriptorReader.java

@@ -194,8 +194,11 @@ private Model loadPom(
                 modelRequest.setValidationLevel(ModelBuildingRequest.VALIDATION_LEVEL_MINIMAL);
                 modelRequest.setProcessPlugins(false);
                 modelRequest.setTwoPhaseBuilding(false);
-                modelRequest.setSystemProperties(toProperties(session.getSystemProperties()));
-                modelRequest.setUserProperties(toProperties(session.getUserProperties()));
+                // This merge is on purpose because otherwise user properties would override model
+                // properties in dependencies the user does not know. See MNG-7563 for details.
+                modelRequest.setSystemProperties(
+                        toProperties(session.getUserProperties(), session.getSystemProperties()));
+                modelRequest.setUserProperties(new Properties());
                 modelRequest.setModelCache(modelCacheFactory.createCache(session));
                 modelRequest.setModelResolver(new DefaultModelResolver(
                         session,
@@ -248,9 +251,14 @@ private Model loadPom(
         }
     }
 
-    private Properties toProperties(Map<String, String> map) {
+    private Properties toProperties(Map<String, String> dominant, Map<String, String> recessive) {
         Properties props = new Properties();
-        props.putAll(map);
+        if (recessive != null) {
+            props.putAll(recessive);
+        }
+        if (dominant != null) {
+            props.putAll(dominant);
+        }
         return props;
     }