Refactor the spring-beans build

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

settings.gradle

@@ -12,7 +12,6 @@ pluginManagement {
 include "spring-aop"
 include "spring-aspects"
 include "spring-beans"
-include "spring-beans-groovy"
 include "spring-context"
 include "spring-context-support"
 include "spring-context-indexer"