fix: 커스텀 어노테이션 + Qualifier로 버그 수정

REF: #5

src/main/java/hello/core/annotation/MainDiscountPolicy.java

@@ -0,0 +1,13 @@
+package hello.core.annotation;
+
+import org.springframework.beans.factory.annotation.Qualifier;
+
+import java.lang.annotation.*;
+
+@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.TYPE, ElementType.ANNOTATION_TYPE})
+@Retention(RetentionPolicy.RUNTIME)
+@Inherited
+@Documented
+@Qualifier("mainDiscountPolicy")
+public @interface MainDiscountPolicy {
+}

src/main/java/hello/core/discount/FixDiscountPolicy.java

@@ -1,10 +1,12 @@
 package hello.core.discount;
 
+import hello.core.annotation.MainDiscountPolicy;
 import hello.core.member.Grade;
 import hello.core.member.Member;
 import org.springframework.stereotype.Component;
 
 @Component
+@MainDiscountPolicy
 public class FixDiscountPolicy implements DiscountPolicy {
 
     private int discountFixAmount = 1000; // 1000원 할인

src/main/java/hello/core/order/OrderServiceImpl.java

@@ -1,9 +1,9 @@
 package hello.core.order;
 
+import hello.core.annotation.MainDiscountPolicy;
 import hello.core.discount.DiscountPolicy;
 import hello.core.member.Member;
 import hello.core.member.MemberRepository;
-import lombok.RequiredArgsConstructor;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.stereotype.Component;
 
@@ -14,7 +14,7 @@ public class OrderServiceImpl implements OrderService {
     private final DiscountPolicy discountPolicy;
 
     @Autowired
-    public OrderServiceImpl(MemberRepository memberRepository, DiscountPolicy discountPolicy) {
+    public OrderServiceImpl(MemberRepository memberRepository, @MainDiscountPolicy DiscountPolicy discountPolicy) {
         this.memberRepository = memberRepository;
         this.discountPolicy = discountPolicy;
     }