[FIXED JENKINS-44898] Add findResource to PluginFirstClassLoader (#2916)

* [FIXED JENKINS-44898] Add findResource to PluginFirstClassLoader

This fixes GroovyClassLoader.loadClass for a .groovy file in a plugin
with a PluginFirstClassLoader, specifically by fixing fast-loading via
the UberClassLoader.

* Move common code to AntWithFindResourceClassLoader.

* Adding testing of new PluginFirstClassLoader behavior.

core/src/main/java/hudson/PluginFirstClassLoader.java

@@ -33,7 +33,7 @@ import java.util.Collection;
 import java.util.Enumeration;
 import java.util.List;
 
-import org.apache.tools.ant.AntClassLoader;
+import jenkins.util.AntWithFindResourceClassLoader;
 
 /**
  * classLoader which use first /WEB-INF/lib/*.jar and /WEB-INF/classes before core classLoader
@@ -42,10 +42,14 @@ import org.apache.tools.ant.AntClassLoader;
  * @since 1.371
  */
 public class PluginFirstClassLoader
-    extends AntClassLoader
+    extends AntWithFindResourceClassLoader
     implements Closeable
 {
-    
+
+    public PluginFirstClassLoader() {
+        super(null, false);
+    }
+
     private List<URL> urls = new ArrayList<URL>();
 
     public void addPathFiles( Collection<File> paths )
@@ -100,6 +104,5 @@ public class PluginFirstClassLoader
     {
         InputStream is = super.getResourceAsStream( name );
         return is;
-    }   
-    
+    }
 }