Add support for loading plugins from extra directories

paper-server/src/main/java/io/papermc/paper/plugin/PluginInitializerManager.java

@@ -116,6 +116,9 @@ public static void load(OptionSet optionSet) throws Exception {
         @SuppressWarnings("unchecked")
         java.util.List<Path> files = ((java.util.List<File>) optionSet.valuesOf("add-plugin")).stream().map(File::toPath).toList();
         io.papermc.paper.plugin.util.EntrypointUtil.registerProvidersFromSource(io.papermc.paper.plugin.provider.source.PluginFlagProviderSource.INSTANCE, files);
+        @SuppressWarnings("unchecked")
+        java.util.List<Path> dirs = ((java.util.List<File>) optionSet.valuesOf("add-plugin-dir")).stream().map(File::toPath).toList();
+        dirs.forEach(pluginDir -> io.papermc.paper.plugin.util.EntrypointUtil.registerProvidersFromSource(io.papermc.paper.plugin.provider.source.DirectoryProviderSource.INSTANCE_NO_CREATE, pluginDir));
 
         final Set<String> paperPluginNames = new TreeSet<>();
         final Set<String> legacyPluginNames = new TreeSet<>();

paper-server/src/main/java/io/papermc/paper/plugin/provider/source/DirectoryProviderSource.java

@@ -16,15 +16,23 @@
  */
 public class DirectoryProviderSource implements ProviderSource<Path, List<Path>> {
 
-    public static final DirectoryProviderSource INSTANCE = new DirectoryProviderSource();
+    public static final DirectoryProviderSource INSTANCE = new DirectoryProviderSource(true);
+    public static final DirectoryProviderSource INSTANCE_NO_CREATE = new DirectoryProviderSource(false);
     private static final FileProviderSource FILE_PROVIDER_SOURCE = new FileProviderSource("Directory '%s'"::formatted, false); // Paper - Remap plugins
     private static final Logger LOGGER = LogUtils.getClassLogger();
 
+    private final boolean createDirectory;
+
+    public DirectoryProviderSource(final boolean createDirectory) {
+        this.createDirectory = createDirectory;
+    }
+
     @Override
     public List<Path> prepareContext(Path context) throws IOException {
         // Symlink happy, create file if missing.
         if (!Files.isDirectory(context)) {
-            Files.createDirectories(context);
+            if(this.createDirectory) Files.createDirectories(context);
+            else return List.of();
         }
 
         final List<Path> files = new ArrayList<>();

paper-server/src/main/java/org/bukkit/craftbukkit/Main.java

@@ -164,6 +164,12 @@ public static void main(String[] args) {
                         .defaultsTo(new File[] {})
                         .describedAs("Jar file");
 
+                this.acceptsAll(asList("add-plugin-dir", "add-extra-plugin-dir"), "Specify paths to extra plugin directories to be loaded in addition to the plugins folder. This argument can be specified multiple times, once for each extra plugin dir path.")
+                    .withRequiredArg()
+                    .ofType(File.class)
+                    .defaultsTo(new File[] {})
+                    .describedAs("Plugin directory");
+
                 this.accepts("server-name", "Name of the server")
                         .withRequiredArg()
                         .ofType(String.class)