RFR: 8152404: Stabilize PackageEntry::package_exports_do

[Rachel Protacio]

Hello,

Please review this bug fix for some module export checking code. The 
PackageEntry::package_exports_do() function had assumed calling 
is_qual_exported() would verify that _qualified_exports was not NULL, 
but as that was not the case, I've re-defined the PackageEntry member 
variables and export checking functions.

Passes JPRT, jck (vm, lang, and api/java_lang), and RBT hotspot_all and 
non-colo tests.

Bug: https://bugs.openjdk.java.net/browse/JDK-8152404
Open webrev: http://cr.openjdk.java.net/~rprotacio/8152404.00/

Thanks,
Rachel