Tweak to PackageResource

[Andrea Del Bene]

Hi,

lately I've spent some time on PackageResource's code while I was 
debugging a problem with a web app. The problem was in my business code 
but I think I've found something that might improve the code and the 
performance of PackageResource. Method 'getCacheKey' (which is called 
multiple times) indirectly calls internalGetResourceStream to check if 
package resource exists, but for this purpose we should use static 
method 'exists' which is lighter than internalGetResourceStream.
I prepared a commit with this change in my GitHub fork: 
https://github.com/bitstorm/wicket/commit/1d75071124d919410d1144911a80cd871ae1325b

WDYT?

Andrea.