JDK 13 RFR of 8216404: Elements.getPackageOf should handle modules

[Joe Darcy]

Hello,

Please review the changes and CSR for

     8216404: Elements.getPackageOf should handle modules
     http://cr.openjdk.java.net/~darcy/8216404.0/
     https://bugs.openjdk.java.net/browse/JDK-8216429

In summary, the method Elements.getPackageOf was not updated to specify 
how modules should be handled. The javac implementation currently throws 
an NPE in that case.

The fix is to add a check for that situation in the javac implementation 
of the Elements interface. This could also be added at the Symbol level, 
but adding it to Elements seemed a more direct approach.

Thanks,

-Joe