Finally, it worked for me, and the reason I think it didn't work earlier was that I had too many dependencies in my pom.xml and things didn't get resolved correctly.
Here is the complete pom.xml I used in order for the code in the original post to work. Rest of the code was used from the aether-demo, the link of which is also provided in the original post
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>aether.example</groupId>
<artifactId>my-aether-plugin</artifactId>
<version>1.0.1-SNAPSHOT</version>
<packaging>maven-plugin</packaging>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<mavenVersion>3.1.1</mavenVersion>
<aetherVersion>0.9.0.M4</aetherVersion>
<mavenPluginVersion>3.2</mavenPluginVersion>
</properties>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-plugin-plugin</artifactId>
<version>${mavenPluginVersion}</version>
<configuration>
<goalPrefix>my-aether</goalPrefix>
<!-- see http://jira.codehaus.org/browse/MNG-5346 -->
<skipErrorNoDescriptorsFound>true</skipErrorNoDescriptorsFound>
</configuration>
<executions>
<execution>
<id>mojo-descriptor</id>
<goals>
<goal>descriptor</goal>
</goals>
</execution>
<execution>
<id>help-goal</id>
<goals>
<goal>helpmojo</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
<dependencies>
<dependency>
<groupId>org.apache.maven</groupId>
<artifactId>maven-plugin-api</artifactId>
<version>${mavenVersion}</version>
<scope>provided</scope>
<exclusions>
<exclusion>
<groupId>org.apache.maven</groupId>
<artifactId>maven-model</artifactId>
</exclusion>
<exclusion>
<groupId>org.apache.maven</groupId>
<artifactId>maven-artifact</artifactId>
</exclusion>
<exclusion>
<groupId>org.eclipse.sisu</groupId>
<artifactId>org.eclipse.sisu.plexus</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>org.eclipse.aether</groupId>
<artifactId>aether-api</artifactId>
<version>${aetherVersion}</version>
</dependency>
<dependency>
<groupId>org.eclipse.aether</groupId>
<artifactId>aether-util</artifactId>
<version>${aetherVersion}</version>
</dependency>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.11</version>
<scope>test</scope>
</dependency>
</dependencies>
</project>