fix: allow adding to mods.main

Author: ishland

src/main/java/net/fabricmc/loom/configuration/CompileConfiguration.java

@@ -152,7 +152,7 @@ public void run() {
 				// Create default mod from main source set
 				extension.mods(mods -> {
 					final SourceSet main = getProject().getExtensions().getByType(JavaPluginExtension.class).getSourceSets().getByName(SourceSet.MAIN_SOURCE_SET_NAME);
-					mods.create("main").sourceSet(main);
+					mods.maybeCreate("main").sourceSet(main);
 				});
 			}
 		});