This implements a bare-bones skeleton for cabal new-exec.

The old cabal exec gave programs access to a sandbox's package database.
By analogy, cabal new-exec should give programs access to the store's
package database; however, this database will be cluttered with many
non-project-related packages that may confuse issues. Therefore new-exec
selects just the packages that are in the current project's dependency
tree and makes them available to compiler tools. Currently only very new
GHCs are supported, via the GHC_ENVIRONMENT mechanism for selecting a
subset of some package databases.

Eventually we should probably also modify the PATH so that dependencies'
executables are available.