gincmd should interface with ginclient only

The top-level commands should only import ginclient (from the subpackages of gin-cli). This means the ginclient will have to expose the functionality of all the other subpackages that might be directly useful.

This change will make the ginclient package a complete layer on top of things like os.Exec, git and git-annex commands, and configuration management. Then the gincmd package will simply be responsible for exposing that functionality to the command line.