Move modules to independent build files

The main `build.gradle` file contains now only the common build
infrastructure; all module-specific build configurations have
been moved to their own build file.

Issue: SPR-15885

buildSrc/spring-build-src.gradle

@@ -0,0 +1,10 @@
+description = "Exposes gradle buildSrc for IDE support"
+
+apply plugin: "groovy"
+
+dependencies {
+	compile gradleApi()
+	compile localGroovy()
+}
+
+configurations.archives.artifacts.clear()