Use YAML compatible classpath.idx format

Update the `classpath.idx` format to align with `layers.idx` and allow
third-parties can parse it as YAML

Closes gh-20861

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

@@ -31,6 +31,8 @@ import java.util.Collections;
 import java.util.List;
 import java.util.stream.Collectors;
 
+import org.springframework.util.Assert;
+
 /**
  * A class path index file that provides ordering information for JARs.
  *
@@ -45,7 +47,12 @@ final class ClassPathIndexFile {
 
 	private ClassPathIndexFile(File root, List<String> lines) {
 		this.root = root;
-		this.lines = lines;
+		this.lines = lines.stream().map(this::extractName).collect(Collectors.toList());
+	}
+
+	private String extractName(String line) {
+		Assert.state(line.startsWith("- \"") && line.endsWith("\""), "Malformed classpath index line [" + line + "]");
+		return line.substring(3, line.length() - 1);
 	}
 
 	int size() {

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/AbstractExecutableArchiveLauncherTests.java

@@ -68,9 +68,9 @@ public abstract class AbstractExecutableArchiveLauncherTests {
 			JarEntry indexEntry = new JarEntry(entryPrefix + "/classpath.idx");
 			jarOutputStream.putNextEntry(indexEntry);
 			Writer writer = new OutputStreamWriter(jarOutputStream, StandardCharsets.UTF_8);
-			writer.write("BOOT-INF/lib/foo.jar\n");
-			writer.write("BOOT-INF/lib/bar.jar\n");
-			writer.write("BOOT-INF/lib/baz.jar\n");
+			writer.write("- \"BOOT-INF/lib/foo.jar\"\n");
+			writer.write("- \"BOOT-INF/lib/bar.jar\"\n");
+			writer.write("- \"BOOT-INF/lib/baz.jar\"\n");
 			writer.flush();
 		}
 		addNestedJars(entryPrefix, "/lib/foo.jar", jarOutputStream);

spring-boot-project/spring-boot-tools/spring-boot-loader/src/test/resources/org/springframework/boot/loader/classpath-index-file.idx

@@ -1,5 +1,5 @@
-a.jar
-b.jar
-c.jar
-d.jar
-e.jar
+- "a.jar"
+- "b.jar"
+- "c.jar"
+- "d.jar"
+- "e.jar"