[JENKINS-33843] Permit disabling optional dependencies (#4001)

* Implied dependencies on detached plugins were not being marked optional despite being listed in optionalDependencies, confusing the calculation of optionalDependents in resolveDependentPlugins.

* Javadoc improvements based on code exploration.

* [JENKINS-33843] Do not consider optional dependencies for purposes of disabling plugins.

* PluginManagerInstalledGUITest updated to reflect new behavior.

core/src/main/java/hudson/PluginManager.java

@@ -1019,15 +1019,18 @@ public abstract class PluginManager extends AbstractModelObject implements OnMas
      * If the war file has any "/WEB-INF/plugins/[*.jpi | *.hpi]", extract them into the plugin directory.
      *
      * @return
-     *      File names of the bundled plugins. Like {"ssh-slaves.hpi","subversion.jpi"}
+     *      File names of the bundled plugins. Normally empty (not to be confused with {@link #loadDetachedPlugins}) but OEM WARs may have some.
      * @throws Exception
      *      Any exception will be reported and halt the startup.
      */
     protected abstract Collection<String> loadBundledPlugins() throws Exception;
 
     /**
-     * Copies the bundled plugin from the given URL to the destination of the given file name (like 'abc.jpi'),
-     * with a reasonable up-to-date check. A convenience method to be used by the {@link #loadBundledPlugins()}.
+     * Copies the plugin from the given URL to the given destination.
+     * Despite the name, this is used also from {@link #loadDetachedPlugins}.
+     * Includes a reasonable up-to-date check.
+     * A convenience method to be used by {@link #loadBundledPlugins()}.
+     * @param fileName like {@code abc.jpi}
      */
     protected void copyBundledPlugin(URL src, String fileName) throws IOException {
         LOGGER.log(FINE, "Copying {0}", src);