hg: jigsaw/jake/jdk: Create new HashSet for packages only if it's different than concealed packages

[mandy.chung at oracle.com]

Changeset: 9c0ea8bde546
Author:    mchung
Date:      2016-01-13 15:58 -0800
URL:       http://hg.openjdk.java.net/jigsaw/jake/jdk/rev/9c0ea8bde546

Create new HashSet for packages only if it's different than concealed packages
to tune startup performance.
Contributed-by: claes.redestad at oracle.com, mandy.chung at oracle.com

! src/java.base/share/classes/java/lang/module/ModuleDescriptor.java
! src/java.base/share/classes/jdk/internal/module/Builder.java