The pro app module should declare contrib modules ids as features

[tmortagne]

The pro and the contrib calendar application cannot work at the same time, but right now they can be installed because the pro app does not indicate anything.

This can create quite a mess, which is enough as a reason to declare the features. But there are also extensions which depend on the contrib calendar macro (because they create events) and since the pro and contrib extensions have compatible metadata it should not be a problem to use those extensions with the pro app but right now you are stuck because you have to install the contrib one (if the pro app was declaring the contrib as feature then it would be found as dependency).