Cleanup ClasspathEntry abstraction

[rwinch]

ClasspathEntry abstraction is not ideal because it often does some work and then provides entries for the classpath. Often times, the ideal situation would be to have a single temporary directory and move all resources to that directory and add that single directory to the classpath rather than a temporary directory per entry. The cleanup is then a single directory that should be deleted rather than needing to perform cleanup on a per ClasspathEntry basis

The other situation is maven which downloads some entries, and adds those jar files directly to the classpath.

For this task we should review what every implementation is doing and then find the commonalities and redefine the abstractions. Perhaps a ClasspathInitializer would be a reasonable abstraction. It might implement Runnable so it can do the resolution in parallel.