Make it easier to subclass Launchers

Change constructors from `package private` to `protected` for subclasses
to use.

Closes gh-3741

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

@@ -55,7 +55,7 @@ public abstract class ExecutableArchiveLauncher extends Launcher {
 		this.javaAgentDetector = javaAgentDetector;
 	}
 
-	ExecutableArchiveLauncher(Archive archive) {
+	protected ExecutableArchiveLauncher(Archive archive) {
 		this.javaAgentDetector = new InputArgumentsJavaAgentDetector();
 		this.archive = archive;
 	}

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

@@ -31,6 +31,13 @@ public class JarLauncher extends ExecutableArchiveLauncher {
 
 	private static final AsciiBytes LIB = new AsciiBytes("lib/");
 
+	public JarLauncher() {
+	}
+
+	protected JarLauncher(Archive archive) {
+		super(archive);
+	}
+
 	@Override
 	protected boolean isNestedArchive(Archive.Entry entry) {
 		return !entry.isDirectory() && entry.getName().startsWith(LIB);

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

@@ -42,7 +42,7 @@ public class WarLauncher extends ExecutableArchiveLauncher {
 		super();
 	}
 
-	WarLauncher(Archive archive) {
+	protected WarLauncher(Archive archive) {
 		super(archive);
 	}