feat: add error log when permalink exists (#2460)

#### What type of PR is this?
/kind improvement
/area core
/milestone 2.0

#### What this PR does / why we need it:
注册 permalink 到 Permalinker 时校验 permalink 是否已经存在，如果存在则记录错误日志且不向 PermalinkIndexer 添加记录

TODO：
- [ ] 后续增加事件功能后检查到 permalink 重复则发送记录事件的 event 来代替日志记录
#### Which issue(s) this PR fixes:

A part of #2454

#### Special notes for your reviewer:
/cc @halo-dev/sig-halo 
#### Does this PR introduce a user-facing change?

```release-note
None
```

src/main/java/run/halo/app/theme/router/PermalinkIndexer.java

@@ -4,6 +4,7 @@ import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.locks.ReentrantReadWriteLock;
+import lombok.extern.slf4j.Slf4j;
 import org.springframework.context.event.EventListener;
 import org.springframework.lang.NonNull;
 import org.springframework.lang.Nullable;
@@ -17,6 +18,7 @@ import run.halo.app.extension.GroupVersionKind;
  * @author guqing
  * @since 2.0.0
  */
+@Slf4j
 @Component
 public class PermalinkIndexer {
     private final ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
@@ -173,8 +175,20 @@ public class PermalinkIndexer {
         return permalinkLocatorLookup.size();
     }
 
+    /**
+     * Add a record to the {@link PermalinkIndexer}.
+     * If permalink already exists, it will not be added to indexer
+     *
+     * @param addCommand a command to add a record to {@link PermalinkIndexer}
+     */
     @EventListener(PermalinkIndexAddCommand.class)
     public void onPermalinkAdd(PermalinkIndexAddCommand addCommand) {
+        if (checkPermalinkExists(addCommand.getLocator(), addCommand.getPermalink())) {
+            // TODO send an extension event to log this error
+            log.error("Permalink [{}] already exists, you can try to change the slug [{}].",
+                addCommand.getPermalink(), addCommand.getLocator());
+            return;
+        }
         register(addCommand.getLocator(), addCommand.getPermalink());
     }
 
@@ -183,9 +197,26 @@ public class PermalinkIndexer {
         remove(deleteCommand.getLocator());
     }
 
+    /**
+     * Update a {@link PermalinkIndexer} record by {@link ExtensionLocator} and permalink.
+     * If permalink already exists, it will not be updated
+     *
+     * @param updateCommand a command to update an indexer record
+     */
     @EventListener(PermalinkIndexUpdateCommand.class)
     public void onPermalinkUpdate(PermalinkIndexUpdateCommand updateCommand) {
+        if (checkPermalinkExists(updateCommand.getLocator(), updateCommand.getPermalink())) {
+            // TODO send an extension event to log this error
+            log.error("Permalink [{}] already exists, you can try to change the slug [{}].",
+                updateCommand.getPermalink(), updateCommand.getLocator());
+            return;
+        }
         remove(updateCommand.getLocator());
         register(updateCommand.getLocator(), updateCommand.getPermalink());
     }
+
+    private boolean checkPermalinkExists(ExtensionLocator locator, String permalink) {
+        ExtensionLocator lookup = lookup(permalink);
+        return lookup != null && !lookup.equals(locator);
+    }
 }