KT-27601 keep imports for extension functions used in kdoc

idea/testData/editor/optimizeImports/jvm/KDocReference.kt

@@ -1,11 +1,16 @@
 import kotlin.concurrent.fixedRateTimer
 import kotlin.concurrent.read
+import kotlin.concurrent.withLock
+import kotlin.concurrent.write
 import java.util.ArrayList
+import java.util.concurrent.locks.Lock
 
 /**
  * Reference to [ArrayList]
  * Reference to [fixedRateTimer]
  * Reference to [kotlin.concurrent.read]
+ * Reference to [Lock.withLock]
+ * Reference to [java.util.concurrent.locks.ReentrantReadWriteLock.write]
  */
 fun foo() {
 }

idea/testData/editor/optimizeImports/jvm/KDocReference.kt.after

@@ -1,5 +1,8 @@
 import java.util.ArrayList
+import java.util.concurrent.locks.Lock
 import kotlin.concurrent.fixedRateTimer
+import kotlin.concurrent.withLock
+import kotlin.concurrent.write
 
 /**
  * Reference to [ArrayList]