Data race when running scripts concurrently

[brutella]

When I run multiple scripts concurrently, I get a data race when the global variables Packages and PackageTypes are accessed here and here.

I add and remove packages from env.Packages and env.PackageTypes before a script is executed. That's why I get a data race because the access to those global variables is not synchronised. I was able to fix this issue by allowing packages to be defined on an environment level – see pull request below.

Might be related to #34

[mattn]

Thanks. Could you please add tests?

[brutella]

Done

[brutella]

@mattn Any chance to get this merged?