This commit simplifies the solver's internal representation of package
conditionals by removing branches that are unreachable.  For example, the solver
now simplifies

if flag(A)
  if flag(A)
    build-depends: package-a
  else
    build-depends: package-b

to

if flag(A)
  build-depends: package-a

because A must be true for the second conditional to be relevant to the
finalized PackageDescription.

This change probably won't improve performance by simplifying real packages.
However, it fixes a bug in the solver's handling of a certain edge case.  Before
this change, it was possible for the solver to create DependencyReasons with
conflicting flag values.  The conflicting values could cause a problem when
one was FlagBoth, the value used when the same dependency appears on both sides
of a conditional.  Here is an example:

if flag(A)
  if flag(A)
    build-depends: unknown-package
  else
    build-depends: unknown-package

Previously, the solver inserted both A=FlagTrue and A=FlagBoth into the
DependencyReason for the dependency on unknown-package. When the previous
commit changed the association list of flag values into a map, FlagBoth replaced
FlagTrue and prevented the solver from trying -A.

This commit simplifies the conditionals to:

if flag(A)
  build-depends: unknown-package

It also adds some unit tests for dependencies occurring on both sides of
conditionals. "testBackjumpingWithCommonDependency" tests the example above.