dita.out.map.htmlhelp.hhk fails to parse maps with <index-see> and <index-see-also>

Here is what the error looks like:

```
[pipeline] [DOTJ013E][ERROR] Failed to parse the referenced file 'GUID-BCAB1F51-5649-4181-90C3-FD4B35940FF0.ditamap'. The XML parser reported the following error:
[pipeline] org.dita.dost.index.TopicrefElement cannot be cast to org.dita.dost.index.IndexTerm
[pipeline] [DOTJ013E][ERROR] Failed to parse the referenced file 'GUID-1533C78F-50B5-425F-879F-EC518291FDF7.ditamap'. The XML parser reported the following error:
[pipeline] org.dita.dost.index.TopicrefElement cannot be cast to org.dita.dost.index.IndexTerm
[pipeline] [DOTJ013E][ERROR] Failed to parse the referenced file 'GUID-46A3E1DF-F0C4-4911-8F4D-CF1DE4FE140F.ditamap'. The XML parser reported the following error:
[pipeline] org.dita.dost.index.TopicrefElement cannot be cast to org.dita.dost.index.IndexTerm
```

The following instances result in the dita.out.map.htmlhelp.hhp target not parsing files:
- The existence of booktitle/keywords/inexterm/index-see (also fails with index-see-also)
- The existence of map/topicref/topicmeta/keywords/indexterm/index-see (also fails with index-see-also)

Thanks for all of your help!

-Josh Johnson