ScriptTemplatesClassRootsIndex: inc version and minor fixes

Content was changed, so we should force reindex.

Minor changes:
- There is no need to save a string, we can just save nothing
 by using the Unit value (see UnitKey.save and UnitKey.read)
- Unit can be shared between all indexes, so we can extract UnitKey object
- It would be good to extract abstract class FileListIndex:
  - for better readability: separate the logic of the specific index from the common FileListIndex implementation
  - someone can use it also
- mapOf (Unit to null) will create singletonMap itself and more readable
- ScriptTemplatesClassRootsIndex.KEY perhaps better to call NAME

idea/scripting-support/test/org/jetbrains/kotlin/idea/script/AbstractScriptTemplatesFromDependenciesTest.kt

@@ -78,8 +78,8 @@ abstract class AbstractScriptTemplatesFromDependenciesTest : HeavyPlatformTestCa
         }
 
         val roots: Collection<VirtualFile> = FileBasedIndex.getInstance().getContainingFiles(
-            ScriptTemplatesClassRootsIndex.KEY,
-            ScriptTemplatesClassRootsIndex.VALUE,
+            ScriptTemplatesClassRootsIndex.NAME,
+            Unit,
             GlobalSearchScope.allScope(project)
         )