Add classpath index support for exploded war archives

Update the Maven and Gradle packaging for war files so that a
`classpath.idx` file is written into the archive that provides the
original order of the classpath, as was previously done for jar files.
The `WarLauncher` class will use this file when running as an exploded
archive to ensure that the classpath order is the same as when running
from the far war.

Fixes gh-19875

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/Layouts.java

@@ -161,6 +161,11 @@ public final class Layouts {
 			return "WEB-INF/classes/";
 		}
 
+		@Override
+		public String getClasspathIndexFileLocation() {
+			return "WEB-INF/classpath.idx";
+		}
+
 		@Override
 		public String getLayersIndexFileLocation() {
 			return "WEB-INF/layers.idx";