This commit merges back the "spring-beans-groovy" module into the main
"spring-beans" one. The build is configured so:
* Java and Groovy sources are jointly compiled
* Kotlin sources are compiled after
With this change, the `MergePlugin` is not used anymore in the project
build and therefore is removed.
The `DetectSplitPackagesPlugin` wasn't applied so it's been removed as
well.
Issue: SPR-15885