[hotfix] Add project root path to files to facilitate compiling modules in child dirs (apache#3063)

Author: Zouxxyy

.mvn/readme.txt

@@ -0,0 +1,4 @@
+The .mvn directory is needed to be able to use the ${maven.multiModuleProjectDirectory} property, since git cannot
+commit an empty directory, add this file.
+
+Once we do not use ${maven.multiModuleProjectDirectory}, we can remove this file and .mvn directory.

pom.xml

@@ -831,18 +831,19 @@ under the License.
                             </importOrder>
 
                             <licenseHeader>
-                                <file>copyright.txt</file>
+                                <!-- replace it with ${project.rootDirectory} after maven 4.0.0, see MNG-7038 -->
+                                <file>${maven.multiModuleProjectDirectory}/copyright.txt</file>
                                 <delimiter>${spotless.delimiter}</delimiter>
                             </licenseHeader>
                         </java>
                         <scala>
                             <scalafmt>
                                 <version>3.4.3</version>
-                                <file>.scalafmt.conf</file>
+                                <file>${maven.multiModuleProjectDirectory}/.scalafmt.conf</file>
                             </scalafmt>
 
                             <licenseHeader>
-                                <file>copyright.txt</file>
+                                <file>${maven.multiModuleProjectDirectory}/copyright.txt</file>
                                 <delimiter>${spotless.delimiter}</delimiter>
                             </licenseHeader>
                         </scala>