Fix worker scripts with cached compilation units

Try to retrieve them from the compilation unit cache instead of from
the file system if possible. The evaluation code can be shared.

In the long run it would be nice to use the type loader here and allow for
worker scripts to have dependencies/imports. But that is a more intrusive
change given the typeloader's dependency on the engine.

Change-Id: I7f9d6be1ff31433d4b14607cf0c25acdf466ac67
Reviewed-by: Lars Knoll <lars.knoll@digia.com>