Allow dedicated build files for subprojects

With this change, each subproject can configure its
build in a dedicated file like:

```
spring-core/spring-core.gradle
spring-web/spring-web.gradle
```

Issue: SPR-15885

settings.gradle

@@ -1,5 +1,3 @@
-rootProject.name = "spring"
-
 include "spring-aop"
 include "spring-aspects"
 include "spring-beans"
@@ -27,3 +25,8 @@ 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"
+}