Unused symbol: do not search for internal member light methods

Use direct references search instead
So #KT-19811 Fixed

idea/testData/inspectionsLocal/unusedSymbol/.inspection

@@ -0,0 +1 @@
+org.jetbrains.kotlin.idea.inspections.UnusedSymbolInspection

idea/testData/inspectionsLocal/unusedSymbol/internal.kt

@@ -0,0 +1,9 @@
+// PROBLEM: none
+
+class Foo {
+    fun test() {
+        abacaba("")
+    }
+
+    internal fun <caret>abacaba(s: String): String = s
+}