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

spring-jcl/spring-jcl.gradle

@@ -0,0 +1,6 @@
+description = "Spring Commons Logging Bridge"
+
+dependencies {
+	optional("org.apache.logging.log4j:log4j-api:${log4jVersion}")
+	optional("org.slf4j:slf4j-api:${slf4jVersion}")
+}