[JENKINS-57528] - Load detached plugins which are implicit deps of other plugins at startup even w/o UC (#4000)

* Do not warn repeatedly of the same missing update site dependency.
This happens when locally testing split plugins (despite their presence in WEB-INF/detached-plugins/*.hpi).

(cherry picked from commit 918a0fd221)

* Detached plugins must be kept up to date with security advisories.

(cherry picked from commit e08d28a698)

* Load detached plugins which are implicit dependencies of other plugins at startup, even if the update center was not used.

(cherry picked from commit ad7a36001d)

* Adjusted expected version after e08d28a698.

(cherry picked from commit 84f939cebd)

* e08d28a698 exposed tests which had been incorrectly storing plugins as *.hpi rather than *.jpi.

(cherry picked from commit a0abad7ec4)

* Test failure after ad7a36001d.

(cherry picked from commit 0084e8dbdd)

* Another test failure after ad7a36001d30a6174b25e616fc3c215dd24919d1; fewer installDetachedPlugin calls are now required.

(cherry picked from commit b65bb54e99)

core/src/main/java/hudson/ClassicPluginStrategy.java

@@ -251,14 +251,18 @@ public class ClassicPluginStrategy implements PluginStrategy {
                 createClassLoader(paths, dependencyLoader, atts), disableFile, dependencies, optionalDependencies);
     }
 
-    private static void fix(Attributes atts, List<PluginWrapper.Dependency> optionalDependencies) {
+    private void fix(Attributes atts, List<PluginWrapper.Dependency> optionalDependencies) {
         String pluginName = atts.getValue("Short-Name");
         
         String jenkinsVersion = atts.getValue("Jenkins-Version");
         if (jenkinsVersion==null)
             jenkinsVersion = atts.getValue("Hudson-Version");
-        
-        optionalDependencies.addAll(DetachedPluginsUtil.getImpliedDependencies(pluginName, jenkinsVersion));
+
+        for (Dependency d : DetachedPluginsUtil.getImpliedDependencies(pluginName, jenkinsVersion)) {
+            LOGGER.fine(() -> "implied dep " + pluginName + " → " + d.shortName);
+            pluginManager.considerDetachedPlugin(d.shortName);
+            optionalDependencies.add(d);
+        }
     }
 
     /**