jdibug doesn't support archives (.zips, .jars) in jde-sourcepath

[GoogleCodeExporter]

What steps will reproduce the problem?
1. attempt to step into a method (eg. java.lang.Object.getClass()) that has 
source contained in, eg. /opt/sun-jdk-1.6.0.26/src.zip 
2. notice jdibug saying something like "JDIbug file java/lang/Object.java not 
in source path".

I see a lot of calls to (file-exists-p filename), and directly 
expanding/scanning the jde-sourcepath entries.  It might be better to defer to 
jde-find-class-source-file, though it has a funny variable return type (either 
a filename or an already-loaded buffer).

Original issue reported on code.google.com by [email protected] on 6 Sep 2011 at 7:20