[CARBONDATA-3888] Move .flattened-pom.xml to target folder

Why is this PR needed?
after .flattened-pom.xml is generated in the project folder, it will impact the project import of Intellij idea

What changes were proposed in this PR?
set outputDirectory of flatten-maven-plugin to project.build.directory

Does this PR introduce any user interface change?
No
Is any new testcase added?
No

This closes #3824
diff --git a/pom.xml b/pom.xml
index 49ad5b5..7f85d0c 100644
--- a/pom.xml
+++ b/pom.xml
@@ -551,6 +551,7 @@
             <artifactId>flatten-maven-plugin</artifactId>
             <version>1.2.2</version>
             <configuration>
+              <outputDirectory>${project.build.directory}</outputDirectory>
             </configuration>
             <executions>
               <!-- enable flattening -->