cabal-install cannot pass package ids to the build process

(Imported from [Trac #937](http://hackage.haskell.org/trac/hackage/ticket/937), reported by @kosmikus on 2012-04-05)

The solver determines an install plan that fixes package ids of installed packages.

However, the Setup interface currently does not allow us to specify package ids once we start building; we can only fix the versions.

This can lead to problems in situations where multiple instances of the same package version are installed on a system, and the install plan selects one, but we cannot uniquely communicate that to the builder.