Hopefully fixes #1725

There are two separate changes here:

1. In the solver, we rewrite the condition trees for packages prior to solving. First, we try to extract from the condition tree of each component the condition under which the component is Buildable. Then we place all build dependencies of the package under the same condition. This should ensure that the solver treats this correctly. Note that we cannot necessarily decide whether a component is Buildable prior to solving, because the conditions can involve automatic flags.
2. When configuring a package (this is in the library), we apply the configuration data and flag assignment in order to collapse down the condition trees of the components. We then also learn whether the components are Buildable or not. We now disregard dependencies for non-buildable components at this point. Fixing this should automatically also fix the sanity check that cabal-install applies to the install plans produced by the solver.