title: Checkstyle Plugin does not add gradle tasks
body: <p>i want to use checkstyle plugin in my gradle project, the gradle documentation says that it will add a few tasks:
  <a href="https://docs.gradle.org/current/userguide/checkstyle_plugin.html" rel="nofollow noreferrer">https://docs.gradle.org/current/userguide/checkstyle_plugin.html</a></p>
  
  <pre><code>checkstyleMain, checkstyleTest, checkstyleSourceSet
  </code></pre>
  
  <p>I added this into my app build.gradle file:</p>
  
  <pre><code>apply plugin: 'checkstyle'
  </code></pre>
  
  <p>I want to run gradle task from cmd to perform code style check, but there are no one checkstyle task. I checked the whole list by typing: </p>
  
  <pre><code>./gradlew tasks
  </code></pre>
  
  <p>I also tried to add checkstyle jar as library dependency to app module.
  Can anyone tell me what i am doing wrong and how can i get my checkstyle tasks?</p>