[FLINK-21445][clients] Refactors ClassPathPackagedProgramRetriever

ClassPathPackagedProgramRetriever is replaced by DefaultPackagedProgramRetriever.
A new interface EntryClassInformationProvider is introduced encapsulating the
extraction of the actual jar/job class information. This enables us to test the
the functionality in a more fine-grained way. Improvements like fail-early were
not added as part of this refactoring to limit the scope of the change.

flink-clients/src/main/java/org/apache/flink/client/deployment/application/EntryClassInformationProvider.java

@@ -0,0 +1,43 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http:https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.flink.client.deployment.application;
+
+import java.io.File;
+import java.util.Optional;
+
+/** {@code EntryClassInformationProvider} provides information about the entry class. */
+public interface EntryClassInformationProvider {
+
+ /**
+ * Returns the {@link File} referring to the Jar file that contains the job class or no {@code
+ * File} if the job class is located on the classpath.
+ *
+ * @return The {@code File} referring to the job's Jar archive or an empty {@code Optional} in
+ * case the job shall be extracted from the classpath.
+ */
+ Optional<File> getJarFile();
+
+ /**
+ * Returns the name of the job class or an empty {@code Optional} if the job class name cannot
+ * be provided.
+ *
+ * @return The name of the job class or an empty {@code Optional}.
+ */
+ Optional<String> getJobClassName();
+}