feature: "stable clone" of repositories for the omnipath about omni

[XaF]

@dynajoe suggested a package approach, which I believe could work for this.

As a refined idea, a package can be referred to as:

path:
  append:
    - package: [email protected]:XaF/omni  # defaults to `cmd` subdir ? or `omni` subdir ?
    - package: [email protected]:XaF/omni
      dir: cmd2
    - /Users/xaf/...
    - path: /Users/xaf/...

When using a package in the path, the repository will be cloned into an omni data directory (along asdf), and that path will be used as the omnipath. This will allow making sure that the local development versions of the users are not affecting omni's behavior.

On top of that, we will also want to:

• support suggesting cloning repositories as a package when going through suggest_clone (do you want to clone xxx as a package ?)
• detect when a omni up-ed repository is a package, and adapt the relative-path-to-absolute-path feature to keep the configuration into package setup (e.g. path: relpath would become path: { package: xxx, dir: relpath } instead of path: abspath