RFR: 8250954: Avoid multiple warnings for external docs with mismatching modularity

[Hannes Wallnoefer]

Please review:

JBS: https://bugs.openjdk.java.net/browse/JDK-8250954
Webrev: http://cr.openjdk.java.net/~hannesw/8250954/webrev.00/

This is a trivial fix to print only one warning per external documentation bundle in case of mismatching modularity, instead of printing one for each contained package.

Thanks,
Hannes