[#1347] Clean up the code

### What's done:

 * `IndentationError` moved to a separate source file.
 * Top-level extensions moved from `IndentationRule.kt` to `StringUtils.kt`.

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

@@ -0,0 +1,7 @@
+package org.cqfn.diktat.ruleset.rules.chapter3.files
+
+/**
+ * @property expected expected indentation as a number of spaces
+ * @property actual actual indentation as a number of spaces
+ */
+internal data class IndentationError(val expected: Int, val actual: Int)

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

@@ -8,8 +8,9 @@ import org.cqfn.diktat.common.config.rules.RulesConfig
 import org.cqfn.diktat.common.config.rules.getRuleConfig
 import org.cqfn.diktat.ruleset.constants.Warnings.WRONG_INDENTATION
 import org.cqfn.diktat.ruleset.rules.DiktatRule
-import org.cqfn.diktat.ruleset.rules.chapter3.files.IndentationRule.Companion.NEWLINE
-import org.cqfn.diktat.ruleset.rules.chapter3.files.IndentationRule.Companion.SPACE
+import org.cqfn.diktat.ruleset.utils.NEWLINE
+import org.cqfn.diktat.ruleset.utils.SPACE
+import org.cqfn.diktat.ruleset.utils.TAB
 import org.cqfn.diktat.ruleset.utils.calculateLineColByOffset
 import org.cqfn.diktat.ruleset.utils.getAllChildrenWithType
 import org.cqfn.diktat.ruleset.utils.getAllLeafsWithSpecificType
@@ -27,6 +28,8 @@ import org.cqfn.diktat.ruleset.utils.indentation.IndentationConfig
 import org.cqfn.diktat.ruleset.utils.indentation.KdocIndentationChecker
 import org.cqfn.diktat.ruleset.utils.indentation.SuperTypeListChecker
 import org.cqfn.diktat.ruleset.utils.indentation.ValueParameterListChecker
+import org.cqfn.diktat.ruleset.utils.isSpaceCharacter
+import org.cqfn.diktat.ruleset.utils.lastIndent
 import org.cqfn.diktat.ruleset.utils.leaveOnlyOneNewLine
 
 import com.pinterest.ktlint.core.ast.ElementType.CALL_EXPRESSION
@@ -506,9 +509,6 @@ class IndentationRule(configRules: List<RulesConfig>) : DiktatRule(
          */
         const val DEFAULT_INDENT_SIZE = 4
         const val NAME_ID = "zct-indentation"
-        internal const val NEWLINE = '\n'
-        internal const val SPACE = ' '
-        internal const val TAB = '\t'
         private val increasingTokens = listOf(LPAR, LBRACE, LBRACKET, LONG_TEMPLATE_ENTRY_START)
         private val decreasingTokens = listOf(RPAR, RBRACE, RBRACKET, LONG_TEMPLATE_ENTRY_END)
         private val matchingTokens = increasingTokens.zip(decreasingTokens)
@@ -558,17 +558,3 @@ class IndentationRule(configRules: List<RulesConfig>) : DiktatRule(
             }
     }
 }
-
-/**
- * @property expected expected indentation as a number of spaces
- * @property actual actual indentation as a number of spaces
- */
-internal data class IndentationError(val expected: Int, val actual: Int)
-
-/**
- * @return indentation of the last line of this string
- */
-internal fun String.lastIndent() = substringAfterLast(NEWLINE).count(::isSpaceCharacter)
-
-private fun isSpaceCharacter(ch: Char): Boolean =
-    ch == SPACE

diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/utils/StringUtils.kt

@@ -6,6 +6,12 @@ package org.cqfn.diktat.ruleset.utils
 
 import org.jetbrains.kotlin.lexer.KtTokens
 
+internal const val NEWLINE = '\n'
+
+internal const val SPACE = ' '
+
+internal const val TAB = '\t'
+
 @Suppress("VARIABLE_NAME_INCORRECT_FORMAT")
 val JAVA = arrayOf("abstract", "assert", "boolean",
     "break", "byte", "case", "catch", "char", "class", "const",
@@ -97,3 +103,15 @@ fun String.removePrefix(): String {
     }
     return this
 }
+
+/**
+ * @return the indentation of the last line of this string.
+ */
+internal fun String.lastIndent() = substringAfterLast(NEWLINE).count(::isSpaceCharacter)
+
+/**
+ * @param ch the character to examine.
+ * @return `true` if [ch] is a [SPACE], `false` otherwise.
+ */
+internal fun isSpaceCharacter(ch: Char): Boolean =
+    ch == SPACE

diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/utils/indentation/Checkers.kt

@@ -5,8 +5,8 @@
 package org.cqfn.diktat.ruleset.utils.indentation
 
 import org.cqfn.diktat.ruleset.rules.chapter3.files.IndentationError
-import org.cqfn.diktat.ruleset.rules.chapter3.files.lastIndent
 import org.cqfn.diktat.ruleset.utils.hasParent
+import org.cqfn.diktat.ruleset.utils.lastIndent
 
 import com.pinterest.ktlint.core.ast.ElementType.ARROW
 import com.pinterest.ktlint.core.ast.ElementType.AS_KEYWORD

diktat-rules/src/test/kotlin/org/cqfn/diktat/util/DiktatRuleSetProvider4Test.kt

@@ -13,6 +13,7 @@ import org.cqfn.diktat.ruleset.rules.DiktatRuleSetProvider
 import com.pinterest.ktlint.core.Rule
 import com.pinterest.ktlint.core.RuleSet
 import com.pinterest.ktlint.core.RuleSetProvider
+import org.assertj.core.api.Assertions.assertThat
 import org.jetbrains.kotlin.com.intellij.lang.ASTNode
 import org.junit.jupiter.api.Assertions
 import org.junit.jupiter.api.Test
@@ -43,6 +44,7 @@ class DiktatRuleSetProviderTest {
             .map { it.nameWithoutExtension }
             .filterNot { it in ignoreFile }
         val rulesName = DiktatRuleSetProvider().get()
+            .asSequence()
             .onEachIndexed { index, rule ->
                 if (index != 0) {
                     Assertions.assertTrue(
@@ -53,8 +55,9 @@ class DiktatRuleSetProviderTest {
             }
             .map { (it as? DiktatRuleSetProvider.OrderedRule)?.rule ?: it }
             .map { it::class.simpleName!! }
-            .filter { it != "DummyWarning" }
-        Assertions.assertEquals(filesName.sorted().toList(), rulesName.sorted())
+            .filterNot { it == "DummyWarning" }
+            .toList()
+        assertThat(rulesName.sorted()).containsExactlyElementsOf(filesName.sorted().toList())
     }
 
     @Test
@@ -122,6 +125,9 @@ class DiktatRuleSetProviderTest {
     }
 
     companion object {
-        private val ignoreFile = listOf("DiktatRuleSetProvider", "DiktatRule")
+        private val ignoreFile = listOf(
+            "DiktatRuleSetProvider",
+            "DiktatRule",
+            "IndentationError")
     }
 }