Ignore duplicate classpath entries, and share classpaths generated from the same URLClassLoader.

This will reduce memory and processing requirements for build systems where many classpath entries
get duplicated.

Patch by: jat, amitmanjhi (pair programmed)
Review by: rjrjr



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@4592 8db76d5a-ed1c-0410-87a9-c151d255dfc7