RFR(XS): 8242134: Consolidate the get_package_entry() in SystemDictionaryShared and ClassLoader

[Calvin Cheung]

JBS: https://bugs.openjdk.java.net/browse/JDK-8242134
webrev: http://cr.openjdk.java.net/~ccheung/jdk15/8242134/webrev.00/

The above proposed change removes get_package_entry() from 
SystemDictionaryShared.
The ClassLoader::get_package_entry() is being modified slightly so that 
it could be used in SystemDictionaryShared.

Passed tier 1 - 4 tests.

thanks,
Calvin