Compute 'UnitId' when we compute a 'ConfiguredPackage';
consequently, we can eliminate 'FakeMap' (finally!)
There is one hack remaining, which is that 'SolverInstallPlan'
gins up fake unit IDs so that it can be keyed on UnitIds.
But this data structure exists only very briefly before
being converted into an 'InstallPlan' or 'ElaboratedInstallPlan'.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>