Allow extensions to load other extension's classes, and store extensions by IDs instead of name (#3946)

- the extensionmanagers `extension` method now takes in a extension id instead of name
- extension folders are now created using extension id's
- Extensions can load classes from other extensions now
- Added warning about external class loading
- Wherever applicable: store extensions internally by id instead of name

api/src/main/java/org/geysermc/geyser/api/extension/ExtensionManager.java

@@ -36,13 +36,13 @@ import java.util.Collection;
 public abstract class ExtensionManager {
 
     /**
-     * Gets an extension with the given name.
+     * Gets an extension by the given ID.
      *
-     * @param name the name of the extension
-     * @return an extension with the given name
+     * @param id the ID of the extension
+     * @return an extension with the given ID
      */
     @Nullable
-    public abstract Extension extension(@NonNull String name);
+    public abstract Extension extension(@NonNull String id);
 
     /**
      * Enables the given {@link Extension}.