Move Gitlab::Shell and Gitlab::ShellAdapter files to lib/

Signed-off-by: Rémy Coutable <remy@rymai.me>

config/initializers/5_backend.rb

-# GIT over SSH
-require_dependency Rails.root.join('lib/gitlab/backend/shell')
-
-# GitLab shell adapter
-require_dependency Rails.root.join('lib/gitlab/backend/shell_adapter')
-
 required_version = Gitlab::VersionInfo.parse(Gitlab::Shell.version_required)
 current_version = Gitlab::VersionInfo.parse(Gitlab::Shell.new.version)