Unpacking should unpack "replace" rules

[nicolas-grekas]

If a pack lists things in its "replace" section, unpacking this pack should copy these rules into the "replace" section of the root composer.json.

I think that implementing this requires patching the Unpacker class only.

Anyone up to give it a try?

[stof]

this also applies to provide

[mehdibo]

@nicolas-grekas is this still needed? I can take a look at it