JDK 12 RFR of JDK-8193462: Fix Filer handling of package-info initial elements

[joe darcy]

Hello,

Swapping in some work from late 2017 
(http://mail.openjdk.java.net/pipermail/compiler-dev/2017-December/011434.html), 
please review a fix to address:

     JDK-8193462: Fix Filer handling of package-info initial elements
     http://cr.openjdk.java.net/~darcy/8193462.1/

This version of the fix incorporates Liam's suggestion to set the source 
file of the package in Enter.

All langtools regression tests pass with the current version of the fix.

Thanks,

-Joe