Cabal previously tried all flag combinations, which was very slow.  The new
algorithm assigns one flag at a time, and backtracks when a flag introduces a
dependency that is unavailable.

The new algorithm handles the Buildable field by adding an extra conditional at
the top level of each component that represents the condition for which the
component is buildable.  Since all dependencies go under the "then" branch, they
are only required when a flag choice makes the component buildable.  The buildable
logic is taken from the cabal-install dependency solver.

This commit also changes the error message that is shown when dependencies are
missing.  Previously, Cabal printed the shortest list of missing dependencies
from a single flag assignment.  Now it takes the union of all dependencies that
caused it to backtrack when trying different combinations of flags, which
requires less searching.