This page describes what is says is a standard Java folder structure, but then demonstrates one that is not the standard structure.

Example:

• On that page: /src/org/something/GroovyClass.groovy
• Maven and Groovy standard: /src/main/groovy/src/org/something/GroovyClass.groovy

In addition, if you use the maven standard structure, @Library does not allow finding of the (groovy) classes underneath and you get:
unable to resolve class