Improve XML schema validation (#18581)

* Fix failing validation in submodules (regression, #18579)
* Skip schema validation when -DskipChecks is set

Signed-off-by: Holger Friedrich <mail@holger-friedrich.de>

pom.xml

@@ -654,7 +654,7 @@ Import-Package: \\
             </validationSet>
           </validationSets>
           <catalogs>
-            <catalog>${session.executionRootDirectory}/src/thing-type-xml-validation-catalog.xml</catalog>
+            <catalog>${basedirRoot}/src/thing-type-xml-validation-catalog.xml</catalog>
           </catalogs>
         </configuration>
         <executions>
@@ -792,6 +792,15 @@ Import-Package: \\
                 </execution>
               </executions>
             </plugin>
+            <plugin>
+              <groupId>org.codehaus.mojo</groupId>
+              <artifactId>xml-maven-plugin</artifactId>
+              <executions>
+                <execution>
+                  <phase>none</phase>
+                </execution>
+              </executions>
+            </plugin>
           </plugins>
         </pluginManagement>
       </build>