cabal-install cannot pass package ids to the build process

[kosmikus]

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.

[kosmikus]

Also see #470.