Add support for unpacking Composer packages

[fabpot]

This PR is short but packed (pun intended) with great features.

Basically, it helps fix an issue people have with Flex: starting a new project.

[Pierstoval]

About the post && unpack feature, I wonder why it's only reserved to the symfony-pack type? Why can't it be associated with the already existing metapackage, which is, by the way, documented?

[nicolas-grekas]

The reason is that when you require several packages at once, only some of them should be unpacked.
composer require a/b a/c b/a b/c --unpack
only those that were designed to be unpacked should be unpacked.

[Pierstoval]

Yeah, but a metapackage is precisely the kind of package that can be unpacked: if a metapackage contains requirements, they should be unpackable. If the metapackage is only here to create conflicts (like symfony/lts or roave/security-advisories), even if you set --unpack, it will not unpack anything, so it can be considered safe.

I'm just commenting on this because symfony-pack will reinforce the idea for some people that Flex is only for Symfony packages, which is not the case. Anyone willing to create a pack should be able to unpack it with Flex even if it's not a Symfony-related package.

[nicolas-grekas]

can != should - that's the point.

it will reinforce the idea for some people that Flex is only for Symfony packages

Flex is only for Symfony apps yes, framework bundle is an implicit requirement of all recipes.
Let's first try improving the Symfony-based experience, and if some ideas are more broad then fine, that's OSS, ppl can contribute to Composer. We cannot fix the world (who are we to even think we could).

Anyone willing to create a pack should be able to unpack it with Flex even if it's not a Symfony-related package

sure, that's the case - the false-ideas are false-ideas, only technical behavior has value here.

[fabpot]

Symfony Flex is currently definitely for Symfony, no questions about it.

[fabpot]

Based on the feedback, unpack never happens recursively. It gives more control. There is now also a new unpack command that allows to unpack already installed packs.