RFR: 8235414: Module level doc-files show "unnamed package" as holder

[Hannes Wallnöfer]

Please review:

JBS: https://bugs.openjdk.java.net/browse/JDK-8235414
Webrev: http://cr.openjdk.java.net/~hannesw/8235414/webrev.00/

I was not sure where to put the test. I could have added it to one of the existing tests but it didn’t seem to fit in anywhere. Since there already is a test for index tags in package files I decided to add a new test for index tags in doc-files next to it.

Hannes