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.

Trace how the solver's install plan reaches the Setup interface and where package versions are currently passed to the build process. Reproduce a case with multiple installed instances of one package version, then verify that the selected package id is communicated unambiguously to the builder.

Written by the indexing model from the issue text.