Refactor: Extract shared exec mixin logic

[vdice]

Per #135 (comment) we should extract shared exec logic (basically, execute a script) into a sanely-named function that all of its actions (Install, Uninstall, Upgrade) can then use -- cuz exec gonna exec no matter what the action is!

[carolynvs]

Now that we have the pkg/exec/builder package, this is done.