feat: always populate MANIFEST with project.name and .version

This pull request directs the plugin to configure the jar task (if present, provided by the java plugin) and/or the war task (if present, provided by the war plugin) to set the Implementation-Version and Implementation-Title attributes in the MANIFEST.MF file included in the result.

The end result is that issue #1 (closed) can be satisfied by pulling out Implementation-Version from the MANIFEST. By using MANIFEST, we use a file in Java specifications and we don't have to pollute the build of downstream projects with our own custom properties file.

Manifest reference:

https://docs.oracle.com/javase/tutorial/deployment/jar/packageman.html

cc: @paul.erickson @apatwary

src/main/groovy/edu/wisc/doit/gradle/ContinuousPublishPlugin.groovy

@@ -2,6 +2,7 @@ package edu.wisc.doit.gradle
 
 import org.gradle.api.Plugin
 import org.gradle.api.Project
+import org.gradle.api.Task
 
 /**
  * Root Gradle {@link Plugin} class.
@@ -19,5 +20,28 @@ class ContinuousPublishPlugin implements Plugin<Project> {
     project.getTasks()
       .create("confirmProjectVersionIncremented",
         ConfirmProjectVersionIncrementedTask.class)
+
+    // findByName can return null; getByName throws UnknownTaskException
+    configureManifest(project.getTasks().findByName("jar"))
+    configureManifest(project.getTasks().findByName("war"))
+  }
+
+  /**
+   * Set 'Implementation-Title' and 'Implementation-Version' attributes
+   * in the task's manifest
+   *
+   * See https://docs.oracle.com/javase/tutorial/deployment/jar/packageman.html
+   *
+   * @param task the task to configure (null safe)
+   */
+  void configureManifest(Task task) {
+    task?.configure {
+      manifest {
+        attributes(
+          'Implementation-Title': project.name,
+          'Implementation-Version': project.version
+        )
+      }
+    }
   }
 }