Move TestSourcesPlugin to a Java Gradle plugin

This commit moves the existing "test sources" Gradle plugin from Groovy
to Java and updates the "buildSrc" build file to prepare for additional
plugins in the Spring Framework build.

The plugin itself looks, for a given Spring Framework module, at all the
project dependencies for the following scopes: "compile", "testCompile",
"api", "implementation" and "optional" (to be supported by a different
plugin).

See gh-23282

settings.gradle

@@ -22,10 +22,6 @@ include "spring-webflux"
 include "spring-websocket"
 include "spring-framework-bom"
 
-// Exposes gradle buildSrc for IDE support
-include "buildSrc"
-rootProject.children.find{ it.name == "buildSrc" }.name = "spring-build-src"
-
 rootProject.name = "spring"
 rootProject.children.each {project ->
 	project.buildFileName = "${project.name}.gradle"