Replace conventions with extension when adding methods to source sets

[tudortimi]

We use conventions to add sv, c, etc. methods to source sets. This is a deprecated feature and we should move away from it.

[tudortimi]

Some notes on how this could be implemented:

https://blog.mrhaki.com/2013/01/groovy-goodness-adding-extra-methods.html https://stackoverflow.com/questions/62318216/how-gradle-plugin-add-method-to-a-specific-block https://github.com/gradle/gradle/blob/master/subprojects/core-api/src/main/java/org/gradle/internal/metaobject/MethodMixIn.java https://www.logicbig.com/tutorials/misc/groovy/method-pointer-operator.html

This needs to be studied in more depth.