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-instrument/spring-instrument.gradle

@@ -0,0 +1,11 @@
+description = "Spring Instrument"
+
+jar {
+	manifest.attributes["Premain-Class"] =
+			"org.springframework.instrument.InstrumentationSavingAgent"
+	manifest.attributes["Agent-Class"] =
+			"org.springframework.instrument.InstrumentationSavingAgent"
+	manifest.attributes["Can-Redefine-Classes"] = "true"
+	manifest.attributes["Can-Retransform-Classes"] = "true"
+	manifest.attributes["Can-Set-Native-Method-Prefix"] = "false"
+}