Fixes compile errors by moving files

The two files moved in this commit are referenced from the
javascript/lib qlpack, but they are located in the
javascript/src qlpack. This causes compile errors when running
compile-ish commands for javascript queries. Moving the
files fixes it.

javascript/ql/src/LanguageFeatures/UnusedIndexVariable.ql

@@ -10,7 +10,7 @@
  */
 
 import javascript
-import UnusedIndexVariable
+import LanguageFeatures.UnusedIndexVariable
 
 from RelationalComparison rel, Variable idx, Variable v
 where unusedIndexVariable(rel, idx, v)