Add command line option to load extra plugin jars not in the plugins folder

ex: java -jar paperclip.jar nogui -add-plugin=/path/to/plugin.jar -add-plugin=/path/to/another/plugin_jar.jar

paper-api/src/main/java/org/bukkit/Bukkit.java

@@ -83,6 +83,20 @@ public final class Bukkit {
         return server;
     }
 
+    /**
+     * Returns the de facto plugins directory, generally used for storing plugin jars to be loaded,
+     * as well as their {@link org.bukkit.plugin.Plugin#getDataFolder() data folders}.
+     *
+     * <p>Plugins should use {@link org.bukkit.plugin.Plugin#getDataFolder()} rather than traversing this
+     * directory manually when determining the location in which to store their data and configuration files.</p>
+     *
+     * @return plugins directory
+     */
+    @NotNull
+    public static File getPluginsFolder() {
+        return server.getPluginsFolder();
+    }
+
     /**
      * Attempts to set the {@link Server} singleton.
      * <p>

paper-api/src/main/java/org/bukkit/Server.java

@@ -68,6 +68,18 @@ import org.jetbrains.annotations.Nullable;
  */
 public interface Server extends PluginMessageRecipient, net.kyori.adventure.audience.ForwardingAudience { // Paper
 
+    /**
+     * Returns the de facto plugins directory, generally used for storing plugin jars to be loaded,
+     * as well as their {@link org.bukkit.plugin.Plugin#getDataFolder() data folders}.
+     *
+     * <p>Plugins should use {@link org.bukkit.plugin.Plugin#getDataFolder()} rather than traversing this
+     * directory manually when determining the location in which to store their data and configuration files.</p>
+     *
+     * @return plugins directory
+     */
+    @NotNull
+    File getPluginsFolder();
+
     /**
      * Used for all administrative messages, such as an operator using a
      * command.

paper-api/src/main/java/org/bukkit/plugin/SimplePluginManager.java

@@ -117,9 +117,22 @@ public final class SimplePluginManager implements PluginManager {
     @Override
     @NotNull
     public Plugin[] loadPlugins(@NotNull File directory) {
+        // Paper start - extra jars
+        return this.loadPlugins(directory, java.util.Collections.emptyList());
+    }
+    @NotNull
+    public Plugin[] loadPlugins(final @NotNull File directory, final @NotNull List<File> extraPluginJars) {
+        // Paper end
         if (true) {
             List<Plugin> pluginList = new ArrayList<>();
             java.util.Collections.addAll(pluginList, this.paperPluginManager.loadPlugins(directory));
+            for (File file : extraPluginJars) {
+                try {
+                    pluginList.add(this.paperPluginManager.loadPlugin(file));
+                } catch (Exception e) {
+                    this.server.getLogger().log(Level.SEVERE, "Plugin loading error!", e);
+                }
+            }
             return pluginList.toArray(new Plugin[0]);
         }
         Preconditions.checkArgument(directory != null, "Directory cannot be null");

paper-api/src/main/java/org/bukkit/plugin/java/JavaPluginLoader.java

@@ -93,7 +93,7 @@ public final class JavaPluginLoader implements PluginLoader {
             throw new InvalidPluginException(ex);
         }
 
-        final File parentFile = file.getParentFile();
+        final File parentFile = this.server.getPluginsFolder(); // Paper
         final File dataFolder = new File(parentFile, description.getName());
         @SuppressWarnings("deprecation")
         final File oldDataFolder = new File(parentFile, description.getRawName());