SPARK-3812 Build changes to publish effective pom.

Author: Prashant Sharma <prashant.s@imaginea.com>

Closes #2921 from ScrapCodes/build-changes-effective-pom and squashes the following commits:

8841491 [Prashant Sharma] Fixed broken maven build.
aa7b91d [Prashant Sharma] used an unused dep.
0300dac [Prashant Sharma] improved comment messages..
28f891e [Prashant Sharma] Added a useless dependency, so that we can shade it. And realized fake shading works for us.
553d96b [Prashant Sharma] Shaded some unused class of an unused dep, to generate effective pom(s)

pom.xml

@@ -248,7 +248,17 @@
       </snapshots>
     </pluginRepository>
   </pluginRepositories>
-
+  <!-- 
+       This is a dummy dependency that is used along with the shading plug-in
+       to create effective poms on publishing (see SPARK-3812).
+  -->
+  <dependencies>
+    <dependency>
+      <groupId>org.spark-project.spark</groupId>
+      <artifactId>unused</artifactId>
+      <version>1.0.0</version>
+    </dependency>
+  </dependencies>
   <dependencyManagement>
     <dependencies>
       <dependency>
@@ -997,6 +1007,27 @@
     </pluginManagement>
 
     <plugins>
+      <!-- The shade plug-in is used here to create effective pom's (see SPARK-3812). -->
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-shade-plugin</artifactId>
+        <configuration>
+          <shadedArtifactAttached>false</shadedArtifactAttached>
+          <artifactSet>
+            <includes>
+              <include>org.spark-project.spark:unused</include>
+            </includes>
+          </artifactSet>
+        </configuration>
+        <executions>
+          <execution>
+            <phase>package</phase>
+            <goals>
+              <goal>shade</goal>
+            </goals>
+          </execution>
+        </executions>
+      </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-enforcer-plugin</artifactId>