Use PsiElement#getResolveScope() instead of custom helper

Fix extensions and types completion completion in code fragments in context of library source files
Also affects completion in files not in our project (not tested)

idea/testData/completion/codeFragmentInLibrarySource/customLibrary/customLibrary.kt

@@ -2,4 +2,7 @@ package customLibrary
 
 public fun foo(parameter: Int): Int {
     return 1
+}
+
+private fun Int.extOnInt() {
 }