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

introspect/structs.py

@@ -8579,11 +8579,11 @@ STRUCTS: Mapping[str, StructDecl] = dict([
                  doc='name',
              ),
              StructFieldDecl(
-                 name='instance_name',
+                 name='plugin_name',
                  type=PointerType(
                      inner_type=ValueType(name='mjString'),
                  ),
-                 doc='instance name',
+                 doc='plugin name',
              ),
              StructFieldDecl(
                  name='plugin_slot',