Introduce SolverId/SolverInstallPlan as solver output.
Currently, dependency solving immediately produces an 'InstallPlan' which is then consumed by cabal install, or elaborated into an 'ElaboratedInstallPlan' for cabal new-build. However, this translation is awkward, because the dependency solver knows nothing about 'UnitId's, yet an 'InstallPlan' must indexed by 'UnitId's. So there's a bit of faffing around to generate a "fake" unit id to satisfy the interface, and then eventually correct it to the right one. So this patch starts moving us in a better direction, by introducing a 'SolverInstallPlan', 'SolverPackage' and 'SolverId', intended to be generated by the solver. Then 'configureInstallPlan' or 'elaborateInstallPlan' elaborate this representation into the representation needed by the destination. The next step will be to generate the 'UnitId' during 'configureInstallPlan', and then we can get rid of the fake map (so only Solver data types generate a fake identity, which is only temporary until we generate 'UnitId's.) Signed-off-by: Edward Z. Yang <firstname.lastname@example.org>