Restructure packages

[akm]

In order to make git-exec easy to test, extract packages from main package.
The packages are

• command
  • Command type to run given command
  • Runner interface and its implementations
• git
  • Functions to use git command
• opts
  • Types and functions fo type safe CLI option definition
  • Using generics to refer concrete Options type
• core
  • main business logic of git-exec
  • Using command, git and opts packages