Update JarLauncher to filter BOOT-INF/classes correctly

Previously, JarLauncher considered any entry whose name began with
BOOT-INF/classes/ as being a nested entry. This was incorrect as it
meant that subdirectories of BOOT-INF/classes/ would be added to the
classpath rather than just BOOT-INF/classes/ itself.

This commit updates JarLauncher so that only directory entries with a
name equal to BOOT-INF/classes are used.

Closes gh-5610

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

@@ -44,7 +44,7 @@ public class JarLauncher extends ExecutableArchiveLauncher {
 	@Override
 	protected boolean isNestedArchive(Archive.Entry entry) {
 		if (entry.isDirectory()) {
-			return entry.getName().startsWith(BOOT_INF_CLASSES);
+			return entry.getName().equals(BOOT_INF_CLASSES);
 		}
 		return entry.getName().startsWith(BOOT_INF_LIB);
 	}