New warning: Unused imports should be removed

### What's done:
* Added new warning
* Added new test

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

@@ -80,10 +80,10 @@ class FileStructureRuleTest : LintTestBase(::FileStructureRule) {
                 |
                 |class Example { 
                 |val x: Test = null
-                |val x: Foo = null
+                |val y: Foo = null
                 |}
             """.trimMargin(),
-            LintError(3, 1, ruleId, "${FILE_UNORDERED_IMPORTS.warnText()} import org.cqfn.diktat.example.Foo...", true)
+            LintError(3, 1, ruleId, "${FILE_UNORDERED_IMPORTS.warnText()} import org.cqfn.diktat.Foo...", true)
         )
     }
 

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

@@ -6,6 +6,8 @@
 
 package org.cqfn.diktat.example
 
-import org.cqfn.diktat.example.Foo
+import org.cqfn.diktat.Foo
 
-class Example
+class Example {
+    val y: Foo = Foo()
+}

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

@@ -10,4 +10,7 @@ import foo.Bar
 /**
  * KDoc
  */
-class Example
+class Example{
+    val x: Bar = Bar()
+    val y: Baz = Baz()
+}

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

@@ -8,4 +8,7 @@ import baz.Baz
 /**
  * KDoc
  */
-class Example
+class Example{
+    val x: Bar = Bar()
+    val y: Baz = Baz()
+}

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

@@ -3,5 +3,7 @@
  */
 @file:JvmName("Foo")
 package org.cqfn.diktat.example
-import org.cqfn.diktat.example.Foo
-class Example
+import org.cqfn.diktat.Foo
+class Example {
+    val y: Foo = Foo()
+}

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

@@ -11,12 +11,14 @@
 package org.cqfn.diktat.example
 
 
-import org.cqfn.diktat.example.Foo
+import org.cqfn.diktat.Foo
 
 
 
 
 
 
 
-class Example
+class Example {
+    val y: Foo = Foo()
+}

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

@@ -5,4 +5,8 @@ import org.cqfn.diktat.example.Bar
 import org.cqfn.diktat.example.Foo
 import org.junit.jupiter.api.Test
 
-class Example
+class Example {
+    val x: Test = Test()
+    val y: Foo = Foo()
+    val z: Bar = Bar()
+}

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

@@ -21,4 +21,12 @@ import java.nio.charset.Charset
 
 import kotlin.system.exitProcess
 
-class Example
+class Example {
+    val x = setOf<Object>(CaseFormat(), Detekt(), Assertions(), Logger(), Bean(), IOException(), URL(), Charset(), ObjectMapper())
+
+    fun Foo() {
+        while (true) {
+            exitProcess(1)
+        }
+    }
+}

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

@@ -17,4 +17,12 @@ import java.nio.charset.Charset
 import io.gitlab.arturbosch.detekt.Detekt
 import org.slf4j.Logger
 
-class Example
+class Example {
+    val x = setOf<Object>(CaseFormat(), Detekt(), Assertions(), Logger(), Bean(), IOException(), URL(), Charset(), ObjectMapper())
+
+    fun Foo() {
+        while (true) {
+            exitProcess(1)
+        }
+    }
+}

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

@@ -4,4 +4,8 @@ import org.junit.jupiter.api.Test
 import org.cqfn.diktat.example.Foo
 // lorem ipsum
 import org.cqfn.diktat.example.Bar
-class Example
+class Example {
+    val x: Test = Test()
+    val y: Foo = Foo()
+    val z: Bar = Bar()
+}