Allow the user to choose a version of a package

[aschempp]

A user should get a list of package versions (similar to the selection in the Contao 3 Composer client).

There should be two options:

1. install version x.y.z and all future maintenance updates
2. install version x.y.z and all future minor updates

The resulting constraint would be

1. ~x.y.z
2. ^x.y.z

This will prevent Composer from downgrading packages, but allows to upgrade according to selection.