Put fixtures outside of src code

[eerison]

I just want to raise up this issue #186 again to know the reason to have the fixtures inside of src code

the first argument that I saw is: it wasn't used for test only!

but in the other hand we do not use this in production code, then composer map those classes that are not useful for production environment.

[greg0ire]

The history looks lost: 073990e1778083280075d613dcca84f847c8fc1a

Not sure where the docs where versioned before this commit. Since there is no argument, please make another proposal, test it on a real project, then send a PR to fix this.

[eerison]

Ok, I'm closing this issue for now, I can try provide a PR later ;)