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() {