Add support to repeaters and luet-repeaters-loader tool

[geaaru]

The idea is to supply a way for users to create a luet packages cache server with only packages used by your installations.

In particular, we want two type of repeaters:

• passive: it's a mirrors to use for try to fetch packages before use the servers defined inside repository urls
• active: it's a server that could be sync through luet-repeaters-loader tool is a similar way:

$> luet search --installed . | luet-repeaters-loader --to myserver1.local --to myserver2.local