Convert both the installed package index and the source package index into one uniform solver index.
We use allPackagesBySourcePackageId for the installed package index
because that returns us several instances of the same package and version
in order of preference. This allows us in principle to "shadow"
packages if there are several installed packages of the same version.
There are currently some shortcomings in both GHC and Cabal in
resolving these situations. However, the right thing to do is to
fix the problem there, so for now, shadowing is only activated if
explicitly requested.