Improve way of header KDOC detection (#974)

### What's done:
* Improve way of header KDOC detection: If kdoc located between package directive and imports list, is also should be moved to the top level
* Add test

diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter3/files/FileStructureRule.kt

@@ -145,7 +145,12 @@ class FileStructureRule(configRules: List<RulesConfig>) : DiktatRule(
             ?.takeIf { blockCommentNode ->
                 copyrightWords.any { blockCommentNode.text.contains(it, ignoreCase = true) }
             }
+        // firstCodeNode could be:
+        // * package directive - in this case we looking for kdoc before package directive
+        // and if it doesn't exist, additionally looking for kdoc before imports list
+        // * imports list or actual code - if there is no kdoc before it, suppose that it is absent in file
         var headerKdoc = firstCodeNode.prevSibling { it.elementType == KDOC }
+            ?: if (firstCodeNode == packageDirectiveNode) importsList?.prevSibling { it.elementType == KDOC } else null
         // Annotations with target`file` can only be placed before `package` directive.
         var fileAnnotations = node.findChildByType(FILE_ANNOTATION_LIST)
         // We also collect all other elements that are placed on top of the file.

diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter3/FileStructureRuleFixTest.kt

@@ -23,6 +23,12 @@ class FileStructureRuleFixTest : FixTestBase("test/paragraph3/file_structure", :
         fixAndCompare("CopyrightCommentPositionExpected.kt", "CopyrightCommentPositionTest.kt")
     }
 
+    @Test
+    @Tag(WarningNames.FILE_INCORRECT_BLOCKS_ORDER)
+    fun `should move header kdoc before package directive`() {
+        fixAndCompare("HeaderKdocAfterPackageExpected.kt", "HeaderKdocAfterPackageTest.kt")
+    }
+
     @Test
     @Tag(WarningNames.FILE_NO_BLANK_LINE_BETWEEN_BLOCKS)
     fun `should insert blank lines between code blocks`() {

diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter3/files/TopLevelOrderRuleFixTest.kt

@@ -20,12 +20,4 @@ class TopLevelOrderRuleFixTest : FixTestBase("test/paragraph3/top_level", ::TopL
     fun `should fix top level order with comment`() {
         fixAndCompare("TopLevelWithCommentExpected.kt", "TopLevelWithCommentTest.kt")
     }
-
-    // FixMe: should be considered this case (swapped order of kdoc and package directive)
-    @Disabled("Isn't working yet, because KDoc is bound to class declaration here. Also, should be moved to FileStructureRuleFixTest.")
-    @Test
-    @Tag(WarningNames.TOP_LEVEL_ORDER)
-    fun `should fix top level order with header kdoc`() {
-        fixAndCompare("TopLevelWithHeaderKdocExpected.kt", "TopLevelWithHeaderKdocTest.kt")
-    }
 }

diktat-rules/src/test/resources/test/paragraph3/file_structure/HeaderKdocAfterPackageExpected.kt

@@ -0,0 +1,11 @@
+/**
+ * Header Kdoc
+ */
+
+package test.paragraph3.file_structure
+
+import org.cqfn.diktat.Bar
+
+class A {
+    val a = Bar()
+}

diktat-rules/src/test/resources/test/paragraph3/file_structure/HeaderKdocAfterPackageTest.kt

@@ -0,0 +1,11 @@
+package test.paragraph3.file_structure
+
+/**
+ * Header Kdoc
+ */
+
+import org.cqfn.diktat.Bar
+
+class A {
+    val a = Bar()
+}