It does not work as intended and gives inconsistent results between
cabal install and cabal configure. The problem with the approach was
that we were filtering out the dependencies of non-buildable components
at the end. But that does not help much since if one of the deps of
the non-buildable component were not available then we would have
failed earlier with a constraint failure. A proper solution would have
to tackle it from the beginning, not just as a filter at the end.
The meaning of build-depends and buildable: False needs more thought.