Make superenv's cc wrapper loadable

The goal here is to make it easier to load this code for testing. This is an OK first step, but one can't `require` the file since it doesn't end in `.rb`, we have to use `load` which is less than ideal (files can be `load`ed multiple times).

I'd prefer to move the `Cmd` class into a `cmd.rb` file (in the same directory, since it is tied to the Xcode version), which would make it `require`able. I doubt requiring a tiny file will noticeably impact performance.