Use packageAt:ifAbsent: in chunk exporter.

#packageAt: is deprecated.

src/Silk-Tests.st

@@ -1,5 +1,5 @@
 Smalltalk createPackage: 'Silk-Tests'!
-(Smalltalk packageAt: 'Silk-Tests') imports: {'amber/jquery/Wrappers-JQuery'}!
+(Smalltalk packageAt: 'Silk-Tests' ifAbsent: [ self error: 'Package not created: Silk-Tests' ]) imports: {'amber/jquery/Wrappers-JQuery'}!
 DOMiteTest subclass: #SilkInheritedTest
 	instanceVariableNames: ''
 	package: 'Silk-Tests'!