ArtifactContext: added script zip for ceylon/ceylon-compiler#1490
JDKUtils: use a map for reverse package->module map for JDK

This was revealed as a costly part of profiling
RepoUsingTool: more methods for ceylon/ceylon-compiler#1490
IOUtils: added zip method for ceylon/ceylon-compiler#1490
IOUtils: don't include leading "/" if path is empty