As a result of the previous InstallPlan refactoring, we can now use the
non-serialisable BuildFailure type from D.C.Types which uses
SomeException, where previously we had to use a copy of that type that
used String for the errors.

So now there's no longer any need to have a separate set of types for
BuildResult, BuildResults, BuildSuccess or BuildFailure. There was a
minor difference in the structure of the BuildSuccess, where in the new
build code we need to be able to produce the InstalledPackageInfo at a
different point from the rest of the info in the BuildSuccess. This can
be kept local to the ProjecBuilding module, but accounts for the
somewhat larger number of changes in that module.