Process `main` source set only

[remal]

We have different kinds of tests in our project:

• unit - test source set
• integration - integrationTest source set
• functional - functionalTest
• etc

How can we process main source set only?

[Nava2]

This seems like it'd be fairly trivial to update the path:
https://github.com/assertj/assertj-generator-gradle-plugin/blob/main/src/main/kotlin/org/assertj/generator/gradle/AssertJGeneratorGradlePlugin.kt#L46-L47

We'd likely need to create a new Extension that is applied to each Project, i.e. https://docs.gradle.org/current/userguide/implementing_gradle_plugins_binary.html#capturing_user_input_to_configure_plugin_runtime_behavior

We probably should only look at main by default. I think that it was an oversight on my behalf to look at more source sets.