Immutables processor writes to generated_src and is not clean friendly

[chris05atm]

What happened?

IntelliJ IDEA 2020.3.1 (Ultimate Edition) Build #IU-203.6682.168, built on December 29, 2020

Running ./gradlew idea generates Immutables annotated files under "${projectDir}/generated_src/. These generated_src files are not cleaned when running ./gradlew clean as expected. I imagine there is a reason this plugin does not write to `"${buildDir}" by default.

What did you want to happen?

If a Gradle plugin generates code it should have an equivalent clean command.