The dependency resolver has had to be extended in a slightly hacky
way to gather the extra information needed by an install plan. In
particular it requires the flags to use to configure each package,
the actual versions of dependencies to use and all of the
installed packages and their closure of dependencies.
However the current resolver is fairly naive and so can be easily
persuaded into producing an invalid install plan, in which case
you'll get a detailed list of reasons as to why it is invalid.