Navigation: Implement proper getOriginalElement() for Kotlin declarations.

Run find usages tests with library for both original and navigation elements. Drop duplicating tests. Simplify test class.
TODO: Some usages are not found yet since light methods built from library files do not retain original declarations

idea/testData/findUsages/librarySources/kotlinLibrary/LibraryFunctionUsages.1.java

@@ -0,0 +1,13 @@
+// PSI_ELEMENT: org.jetbrains.kotlin.psi.JetNamedFunction
+// OPTIONS: usages
+// FIND_BY_REF
+// WITH_FILE_NAME
+package usages
+
+import library.*
+
+class J {
+    static void test() {
+        LibraryPackage.foo();
+    }
+}