With this change we can now include all parents of the script, which will make
relative imports always work.
Now the whole meta_path is scanned and not just importlib's PathFinder.
Fixes#1183.
- add sys_path= kwarg to Script & Evaluator constructors
- store sys_path for each evaluator instance
- replace get_sys_path with get_venv_path
- get_venv_path: use addsitedir to load .pth extension files
- get_venv_path: look for egg-link files in all directories in path
