Fix extra kotlin source set structure.

There was an error attaching SourceSet for Kotlin. It caused troubles when customising Kotlin source location. It required extra `kotlin` block
Before the fix
```groovy
sourceSets {
        main {
             kotlin {
                kotlin {
                    srcDir 'customLocation/source'
                 }
              }
          }
       }
```
After the fix
```groovy
    sourceSets {
        main {
            kotlin {
                 srcDir 'customLocation/source'
              }
          }
       }
```