Skip synthetic parameter fix on non-obfuscated classes. (#1463)

* Skip synthetic parameter fixes on obfuscated classes.

This only applies to classes that proguard renamed.

* Keep it simple

src/main/java/net/fabricmc/loom/configuration/providers/minecraft/MinecraftJarMerger.java

@@ -226,7 +226,7 @@ public class MinecraftJarMerger implements AutoCloseable {
 						visitor = new SnowmanClassVisitor(Constants.ASM_VERSION, visitor);
 					}
 
-					if (offsetSyntheticsParams) {
+					if (offsetSyntheticsParams && !entry.contains("/")) {
 						visitor = new SyntheticParameterClassVisitor(Constants.ASM_VERSION, visitor);
 					}