Avoid changing semantics of mjsPlugin.name across XML sections.

The `extension` section used to discard the plugin name and write the instance name in `plugin.name`, while all of the other sections wrote the instance name in `plugin.instance_name` and optionally the plugin name in `plugin.name` (via the function `OnePlugin`). Now all plugin names are in `plugin.plugin_name` and the instance is written in `plugin.name`, so that it is possible to lookup a plugin via its unique instance name.

PiperOrigin-RevId: 683183804
Change-Id: I179ef5d221143b720db24d745b09e327bf1449ef

src/user/user_objects.h

@@ -1412,7 +1412,7 @@ class mjCPlugin_ : public mjCBase {
   std::vector<char> flattened_attributes;  // config attributes flattened in plugin-declared order;
 
  protected:
-  std::string instance_name;
+  std::string plugin_name;
 };
 
 class mjCPlugin : public mjCPlugin_ {
@@ -1428,7 +1428,6 @@ class mjCPlugin : public mjCPlugin_ {
 
  private:
   void Compile(void);              // compiler
-  void NameSpace(const mjCModel* m);
 };